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/store/signup_request.go

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package store
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+var ErrSignupRequestExists = errors.New("signup request already exists")
+
+type SignupRequestStore struct {
+	db *sql.DB
+}
+
+func NewSignupRequestStore(db *sql.DB) *SignupRequestStore {
+	return &SignupRequestStore{db: db}
+}
+
+// Create stores a pending signup request with a hashed password.
+func (s *SignupRequestStore) Create(username, password string) error {
+	hash, err := hashPassword(password)
+	if err != nil {
+		return fmt.Errorf("hash password: %w", err)
+	}
+
+	_, err = s.db.Exec(
+		`INSERT INTO signup_requests (username, password_hash) VALUES (?, ?)`,
+		username, hash,
+	)
+	if err != nil {
+		if strings.Contains(err.Error(), "UNIQUE constraint failed") {
+			return ErrSignupRequestExists
+		}
+		return fmt.Errorf("insert signup request: %w", err)
+	}
+	return nil
+}
+
+// PendingCount returns the number of pending signup requests.
+func (s *SignupRequestStore) PendingCount() (int, error) {
+	var n int
+	err := s.db.QueryRow("SELECT COUNT(*) FROM signup_requests WHERE status = 'pending'").Scan(&n)
+	return n, err
+}