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/context.go

@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package middleware
+
+import (
+	"context"
+	"net/http"
+
+	"kode.naiv.no/olemd/favoritter/internal/model"
+)
+
+type contextKey string
+
+const (
+	userKey      contextKey = "user"
+	csrfTokenKey contextKey = "csrf_token"
+	realIPKey    contextKey = "real_ip"
+)
+
+// UserFromContext returns the authenticated user from the request context, or nil.
+func UserFromContext(ctx context.Context) *model.User {
+	u, _ := ctx.Value(userKey).(*model.User)
+	return u
+}
+
+// CSRFTokenFromContext returns the CSRF token from the request context.
+func CSRFTokenFromContext(ctx context.Context) string {
+	s, _ := ctx.Value(csrfTokenKey).(string)
+	return s
+}
+
+// RealIPFromContext returns the real client IP from the request context.
+func RealIPFromContext(ctx context.Context) string {
+	s, _ := ctx.Value(realIPKey).(string)
+	return s
+}
+
+// RequireLogin redirects to the login page if no user is authenticated.
+func RequireLogin(basePath string) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if UserFromContext(r.Context()) == nil {
+				http.Redirect(w, r, basePath+"/login", http.StatusSeeOther)
+				return
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
+// RequireAdmin returns 403 if the user is not an admin.
+func RequireAdmin(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		user := UserFromContext(r.Context())
+		if user == nil || !user.IsAdmin() {
+			http.Error(w, "Forbidden", http.StatusForbidden)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}