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.go

@@ -47,26 +47,34 @@ func ConfigFromEnv() Config {
 // Configured reports whether accounts can actually be provisioned.
 func (c Config) Configured() bool { return c.BaseURL != "" && c.Token != "" }
 
+// LoginURL is the web sign-in page members are pointed at in their welcome
+// email, e.g. https://git.profullstack.com/user/login.
+func (c Config) LoginURL() string {
+	return strings.TrimRight(c.BaseURL, "/") + "/user/login"
+}
+
 // EnsureUser creates a Forgejo account for username (forwarding to email) if it
-// does not already exist. It is idempotent: created is false when the account
-// was already present. New accounts are created with must_change_password — git
-// access is via SSH keys, so the generated password is never used interactively.
-func (c Config) EnsureUser(username, email string) (created bool, err error) {
+// does not already exist. It is idempotent: created is false (and password "")
+// when the account was already present. New accounts get a generated temporary
+// password with must_change_password set; the caller emails it to the member so
+// they can sign in to the web UI once and set their own. Git over SSH still uses
+// their registered key.
+func (c Config) EnsureUser(username, email string) (created bool, password string, err error) {
 	if !c.Configured() {
-		return false, fmt.Errorf("forgejo not configured")
+		return false, "", fmt.Errorf("forgejo not configured")
 	}
 
 	exists, err := c.userExists(username)
 	if err != nil {
-		return false, err
+		return false, "", err
 	}
 	if exists {
-		return false, nil
+		return false, "", nil
 	}
 
 	pw, err := randomPassword()
 	if err != nil {
-		return false, err
+		return false, "", err
 	}
 	body, _ := json.Marshal(map[string]any{
 		"username":             username,
@@ -76,12 +84,62 @@ func (c Config) EnsureUser(username, email string) (created bool, err error) {
 	})
 	status, resp, err := c.do(http.MethodPost, "/admin/users", body)
 	if err != nil {
-		return false, err
+		return false, "", err
 	}
 	if status < 200 || status >= 300 {
-		return false, fmt.Errorf("forgejo create user %q: %d: %s", username, status, truncate(resp, 200))
+		return false, "", fmt.Errorf("forgejo create user %q: %d: %s", username, status, truncate(resp, 200))
 	}
-	return true, nil
+	return true, pw, nil
+}
+
+// EnsureUserReset creates the account if missing, or resets an existing
+// account's password to a fresh temporary one with must_change_password set.
+// Unlike EnsureUser it always returns a usable password — even for accounts
+// that already exist (whose original one-time password we no longer hold).
+// created reports whether the account was newly made. Used by the notify-creds
+// re-send so every member receives working web credentials.
+func (c Config) EnsureUserReset(username, email string) (created bool, password string, err error) {
+	if !c.Configured() {
+		return false, "", fmt.Errorf("forgejo not configured")
+	}
+	exists, err := c.userExists(username)
+	if err != nil {
+		return false, "", err
+	}
+	pw, err := randomPassword()
+	if err != nil {
+		return false, "", err
+	}
+	if !exists {
+		body, _ := json.Marshal(map[string]any{
+			"username":             username,
+			"email":                email,
+			"password":             pw,
+			"must_change_password": true,
+		})
+		status, resp, err := c.do(http.MethodPost, "/admin/users", body)
+		if err != nil {
+			return false, "", err
+		}
+		if status < 200 || status >= 300 {
+			return false, "", fmt.Errorf("forgejo create user %q: %d: %s", username, status, truncate(resp, 200))
+		}
+		return true, pw, nil
+	}
+	// Reset the existing account's password. login_name/source_id are optional
+	// for local accounts in current Forgejo, so we send only the fields we change.
+	body, _ := json.Marshal(map[string]any{
+		"password":             pw,
+		"must_change_password": true,
+	})
+	status, resp, err := c.do(http.MethodPatch, "/admin/users/"+username, body)
+	if err != nil {
+		return false, "", err
+	}
+	if status < 200 || status >= 300 {
+		return false, "", fmt.Errorf("forgejo reset user %q: %d: %s", username, status, truncate(resp, 200))
+	}
+	return false, pw, nil
 }
 
 // EnsureKey registers an SSH public key on the member's Forgejo account so the