Email members their git + mailbox credentials on provisioning (#57)

* feat(files): SFTP member storage — private workspaces + shared public area + mgmt TUI

Implements M4 (Files). A fully virtual Go SFTP server (pkg/sftp + crypto/ssh,
no OS users) wired as an "sftp" subsystem on the existing :22 wish listener, so
members reach their files with their login key:

    sftp files@bbs.profullstack.com     # scp/rsync ride the same endpoint

Identity is the SSH key (the username is conventional/ignored). Two areas per
session: a private, quota-limited /me workspace and a single shared public file
area /public (old-school BBS file area; world-read, members-only write by
default, operator-moderated). This reverses the old NG1 "no sharing" boundary in
favour of one sanctioned, inspectable sharing surface (PRD §9.3 amended).

internal/files:
- backend.go  service, layout, quota/usage, live-session registry, operator API
- fs.go       per-session virtual FS; resolve() is the single security
              chokepoint (area confinement + symlink-escape guard) + pkg/sftp
              request handlers
- server.go   subsystem handler: key auth -> member session -> request server,
              with byte metering and force-disconnect
- tui.go      in-BBS member browser (hub plugin "Files")
- admin.go    operator management TUI: sessions, workspaces/quotas, public area

Operator console: ssh sftp@<host> (allowlist-gated; sftpadmin@/filesadmin@
aliases) — list/disconnect sessions, set per-user quotas, revoke SFTP access,
toggle public write, moderate the public area.

store: files_access (per-user quota override + revoked) and files_settings
(public-write mode) tables + methods. main.go wiring guarded by AGENTBBS_FILES
(+ AGENTBBS_FILES_QUOTA_MB, default 1 GiB). Route names reserved.

Tests (incl -race): path traversal/confinement, symlink-escape rejection,
public-write ACL, quota enforcement, usage accounting, and an end-to-end run
against a real SFTP client. Docs: docs/files.md; PRD §5.3/§5.3.1/§9.3 + README
updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Add notify-creds subcommand to (re)email members git + mailbox creds

`agentbbs notify-creds` backfills credential emails to verified members
who signed up before the git/mailbox welcome emails existed.

- git (all verified): forgejo.EnsureUserReset resets each account to a
  fresh one-time password (must-change) and emails the web login link,
  username, and password. New method since the original one-time
  password is not recoverable for existing accounts.
- mailbox (all verified): ensures the forwardemail alias and emails the
  address + webmail link.
- Preview by default; --send executes. --git/--mail/--user filters.
  Refuses --send without SMTP; warns+skips when Forgejo/forwardemail
  are unconfigured.

Also folds in the welcome-email functions (gitWelcomeEmailBody,
mailWelcomeEmailBody, EnsureUser password return, provisionGit/
ensurePremium sends) that this builds on. README ops + forgejo tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

internal/forgejo/forgejo_test.go

@@ -20,7 +20,7 @@ func TestConfiguredRequiresURLAndToken(t *testing.T) {
 }
 
 func TestEnsureUserNoOpWhenUnconfigured(t *testing.T) {
-	if _, err := (Config{}).EnsureUser("alice", "a@x.com"); err == nil {
+	if _, _, err := (Config{}).EnsureUser("alice", "a@x.com"); err == nil {
 		t.Fatal("expected error when unconfigured")
 	}
 }
@@ -52,13 +52,19 @@ func TestEnsureUserCreatesWhenMissing(t *testing.T) {
 	defer srv.Close()
 
 	c := Config{BaseURL: srv.URL, Token: "secret"}
-	created, err := c.EnsureUser("alice", "a@x.com")
+	created, password, err := c.EnsureUser("alice", "a@x.com")
 	if err != nil {
 		t.Fatalf("EnsureUser: %v", err)
 	}
 	if !created {
 		t.Fatal("expected created=true")
 	}
+	if password == "" {
+		t.Fatal("expected a generated temporary password for a new account")
+	}
+	if password != got.body["password"] {
+		t.Errorf("returned password %q does not match the one sent to forgejo %v", password, got.body["password"])
+	}
 	if !got.lookup || !got.create {
 		t.Fatalf("expected lookup+create, got %+v", got)
 	}
@@ -70,6 +76,78 @@ func TestEnsureUserCreatesWhenMissing(t *testing.T) {
 	}
 }
 
+func TestEnsureUserResetCreatesWhenMissing(t *testing.T) {
+	var posted bool
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch {
+		case r.Method == http.MethodGet && r.URL.Path == "/api/v1/users/alice":
+			w.WriteHeader(http.StatusNotFound)
+		case r.Method == http.MethodPost && r.URL.Path == "/api/v1/admin/users":
+			posted = true
+			w.WriteHeader(http.StatusCreated)
+			_, _ = w.Write([]byte(`{"id":1}`))
+		default:
+			t.Errorf("unexpected %s %s", r.Method, r.URL.Path)
+			w.WriteHeader(http.StatusTeapot)
+		}
+	}))
+	defer srv.Close()
+
+	c := Config{BaseURL: srv.URL, Token: "secret"}
+	created, password, err := c.EnsureUserReset("alice", "a@x.com")
+	if err != nil {
+		t.Fatalf("EnsureUserReset: %v", err)
+	}
+	if !created || password == "" || !posted {
+		t.Fatalf("expected created+password+POST, got created=%v pw=%q posted=%v", created, password, posted)
+	}
+}
+
+func TestEnsureUserResetPatchesWhenExists(t *testing.T) {
+	var patched bool
+	var body map[string]any
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch {
+		case r.Method == http.MethodGet && r.URL.Path == "/api/v1/users/alice":
+			w.WriteHeader(http.StatusOK)
+			_, _ = w.Write([]byte(`{"id":1}`))
+		case r.Method == http.MethodPatch && r.URL.Path == "/api/v1/admin/users/alice":
+			patched = true
+			_ = json.NewDecoder(r.Body).Decode(&body)
+			w.WriteHeader(http.StatusOK)
+			_, _ = w.Write([]byte(`{"id":1}`))
+		case r.Method == http.MethodPost:
+			t.Error("must not create when the account already exists")
+			w.WriteHeader(http.StatusTeapot)
+		default:
+			t.Errorf("unexpected %s %s", r.Method, r.URL.Path)
+			w.WriteHeader(http.StatusTeapot)
+		}
+	}))
+	defer srv.Close()
+
+	c := Config{BaseURL: srv.URL, Token: "secret"}
+	created, password, err := c.EnsureUserReset("alice", "a@x.com")
+	if err != nil {
+		t.Fatalf("EnsureUserReset: %v", err)
+	}
+	if created {
+		t.Fatal("expected created=false for existing account")
+	}
+	if password == "" {
+		t.Fatal("expected a fresh password even for an existing account")
+	}
+	if !patched {
+		t.Fatal("expected a PATCH to reset the password")
+	}
+	if body["password"] != password {
+		t.Errorf("returned password %q does not match the one sent %v", password, body["password"])
+	}
+	if body["must_change_password"] != true {
+		t.Errorf("expected must_change_password=true, got %v", body["must_change_password"])
+	}
+}
+
 func TestEnsureUserNoOpWhenExists(t *testing.T) {
 	created := false
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -82,13 +160,16 @@ func TestEnsureUserNoOpWhenExists(t *testing.T) {
 	defer srv.Close()
 
 	c := Config{BaseURL: srv.URL, Token: "secret"}
-	got, err := c.EnsureUser("alice", "a@x.com")
+	got, password, err := c.EnsureUser("alice", "a@x.com")
 	if err != nil {
 		t.Fatalf("EnsureUser: %v", err)
 	}
 	if got {
 		t.Fatal("expected created=false for existing user")
 	}
+	if password != "" {
+		t.Fatal("expected empty password when account already exists")
+	}
 	if created {
 		t.Fatal("must not POST when the user already exists")
 	}