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/basepath.go

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package middleware
+
+import (
+	"net/http"
+	"strings"
+)
+
+// BasePath strips a configured path prefix from incoming requests so the
+// router sees paths without the prefix. This enables deployment at both
+// faves.example.com (basePath="") and example.com/faves (basePath="/faves").
+func BasePath(prefix string) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		if prefix == "" {
+			return next
+		}
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			// Strip the prefix from the URL path.
+			if strings.HasPrefix(r.URL.Path, prefix) {
+				r.URL.Path = strings.TrimPrefix(r.URL.Path, prefix)
+				if r.URL.Path == "" {
+					r.URL.Path = "/"
+				}
+				// Also update RawPath if present.
+				if r.URL.RawPath != "" {
+					r.URL.RawPath = strings.TrimPrefix(r.URL.RawPath, prefix)
+					if r.URL.RawPath == "" {
+						r.URL.RawPath = "/"
+					}
+				}
+			}
+			next.ServeHTTP(w, r)
+		})
+	}
+}