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/security.go

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package middleware
+
+import "net/http"
+
+// SecurityHeaders adds security-related HTTP headers to every response.
+func SecurityHeaders(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		h := w.Header()
+		h.Set("X-Content-Type-Options", "nosniff")
+		h.Set("X-Frame-Options", "DENY")
+		h.Set("Referrer-Policy", "strict-origin-when-cross-origin")
+		h.Set("Permissions-Policy", "camera=(), microphone=(), geolocation=()")
+		// CSP allows inline styles from Pico CSS and scripts from self only.
+		// The 'unsafe-inline' for style-src is needed for Pico CSS.
+		h.Set("Content-Security-Policy",
+			"default-src 'self'; "+
+				"style-src 'self' 'unsafe-inline'; "+
+				"img-src 'self' data:; "+
+				"frame-ancestors 'none'")
+		next.ServeHTTP(w, r)
+	})
+}