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>

web/templates/pages/login.html

@@ -0,0 +1,24 @@
+{{define "head"}}
+<meta name="robots" content="noindex">
+{{end}}
+
+{{define "content"}}
+<article>
+    <h1>Logg inn</h1>
+    <form method="POST" action="{{basePath}}/login">
+        <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
+        <label for="username">
+            Brukernavn
+            <input type="text" id="username" name="username" required autofocus
+                   autocomplete="username" autocapitalize="none">
+        </label>
+        <label for="password">
+            Passord
+            <input type="password" id="password" name="password" required
+                   autocomplete="current-password">
+        </label>
+        <button type="submit">Logg inn</button>
+    </form>
+    <p><a href="{{basePath}}/signup">Har du ikke konto? Registrer deg</a></p>
+</article>
+{{end}}