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/logger.go

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package middleware
+
+import (
+	"log/slog"
+	"net/http"
+	"time"
+)
+
+// responseWriter wraps http.ResponseWriter to capture the status code.
+type responseWriter struct {
+	http.ResponseWriter
+	statusCode int
+}
+
+func (rw *responseWriter) WriteHeader(code int) {
+	rw.statusCode = code
+	rw.ResponseWriter.WriteHeader(code)
+}
+
+// RequestLogger logs each HTTP request with method, path, status, and duration.
+func RequestLogger(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		start := time.Now()
+		rw := &responseWriter{ResponseWriter: w, statusCode: http.StatusOK}
+
+		next.ServeHTTP(rw, r)
+
+		slog.Debug("request",
+			"method", r.Method,
+			"path", r.URL.Path,
+			"status", rw.statusCode,
+			"duration", time.Since(start),
+			"ip", RealIPFromContext(r.Context()),
+		)
+	})
+}