feat: implement Phase 1 (auth) and Phase 2 (faves CRUD) foundation

Go backend with server-rendered HTML/HTMX frontend, SQLite database,
and filesystem image storage. Self-hostable single-binary architecture.

Phase 1 — Authentication & project foundation:
- Argon2id password hashing with timing-attack prevention
- Session management with cookie-based auth and periodic cleanup
- Login, signup (open/requests/closed modes), logout, forced password reset
- CSRF double-submit cookie pattern with HTMX auto-inclusion
- Proxy-aware real IP extraction (WireGuard/Tailscale support)
- Configurable base path for subdomain and subpath deployment
- Rate limiting on auth endpoints with background cleanup
- Security headers (CSP, X-Frame-Options, Referrer-Policy)
- Structured logging with slog, graceful shutdown
- Pico CSS + HTMX vendored and embedded via go:embed

Phase 2 — Faves CRUD with tags and images:
- Full CRUD for favorites with ownership checks
- Image upload with EXIF stripping, resize to 1920px, UUID filenames
- Tag system with HTMX autocomplete (prefix search, popularity-sorted)
- Privacy controls (public/private per fave, user-configurable default)
- Tag browsing, pagination, batch tag loading (avoids N+1)
- OpenGraph meta tags on public fave detail pages

Includes code quality pass: extracted shared helpers, fixed signup
request persistence bug, plugged rate limiter memory leak, removed
dead code, and logged previously-swallowed errors.

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

internal/middleware/auth.go

@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package middleware
+
+import (
+	"context"
+	"errors"
+	"net/http"
+
+	"kode.naiv.no/olemd/favoritter/internal/store"
+)
+
+const SessionCookieName = "session"
+
+// ClearSessionCookie sets an expired session cookie to remove it from the client.
+func ClearSessionCookie(w http.ResponseWriter) {
+	http.SetCookie(w, &http.Cookie{
+		Name:     SessionCookieName,
+		Value:    "",
+		Path:     "/",
+		MaxAge:   -1,
+		HttpOnly: true,
+	})
+}
+
+// SessionLoader loads the user from the session cookie on every request.
+// If the session is valid, the user is attached to the request context.
+func SessionLoader(sessions *store.SessionStore, users *store.UserStore) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			cookie, err := r.Cookie(SessionCookieName)
+			if err != nil {
+				next.ServeHTTP(w, r)
+				return
+			}
+
+			session, err := sessions.Validate(cookie.Value)
+			if err != nil {
+				if errors.Is(err, store.ErrSessionNotFound) {
+					ClearSessionCookie(w)
+				}
+				next.ServeHTTP(w, r)
+				return
+			}
+
+			user, err := users.GetByID(session.UserID)
+			if err != nil || user.Disabled {
+				sessions.Delete(cookie.Value)
+				ClearSessionCookie(w)
+				next.ServeHTTP(w, r)
+				return
+			}
+
+			ctx := context.WithValue(r.Context(), userKey, user)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}