feat: add profiles, public views, settings, and code quality fixes

Phase 3 — Profiles & Public Views:
- Public profile page (/u/{username}) with OG meta tags
- User settings page (display name, bio, visibility, default privacy)
- Avatar upload with image processing
- Password change from settings (verifies current password)
- Home page shows public fave feed for logged-in users
- Must-reset-password guard redirects to /reset-password
- Profile visibility: public (full) or limited (username only)

Code quality improvements from /simplify review:
- Fix signup request persistence bug (was silently discarding data)
- Fix health check to use configured listen address, not hardcoded :8080
- Add rate limiter cleanup goroutine (was leaking memory)
- Extract shared helpers: ClearSessionCookie, IsSecureRequest, scanTags,
  scanUserFrom (scanner interface), SignupRequestStore
- Replace hand-rolled joinPlaceholders with strings.Join
- Remove dead _method hidden field, redundant devMode field
- Simplify rate-limited route registration (remove double-mux)
- Log previously-swallowed errors (session delete, image delete)
- Stop leaking internal error messages to users in image upload

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/middleware/resetguard.go

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package middleware
+
+import (
+	"net/http"
+	"strings"
+)
+
+// MustResetPasswordGuard redirects users who must reset their password
+// to the reset page. Allows through: static assets, health, logout,
+// and the reset-password page itself.
+func MustResetPasswordGuard(basePath string) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			user := UserFromContext(r.Context())
+			if user != nil && user.MustResetPassword {
+				path := r.URL.Path
+				// Allow these paths through without redirect.
+				if path == "/reset-password" ||
+					path == "/logout" ||
+					path == "/health" ||
+					strings.HasPrefix(path, "/static/") {
+					next.ServeHTTP(w, r)
+					return
+				}
+				http.Redirect(w, r, basePath+"/reset-password", http.StatusSeeOther)
+				return
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}