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/settings.go

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package store
+
+import (
+	"database/sql"
+	"fmt"
+	"time"
+
+	"kode.naiv.no/olemd/favoritter/internal/model"
+)
+
+type SettingsStore struct {
+	db *sql.DB
+}
+
+func NewSettingsStore(db *sql.DB) *SettingsStore {
+	return &SettingsStore{db: db}
+}
+
+// Get returns the current site settings.
+func (s *SettingsStore) Get() (*model.SiteSettings, error) {
+	var settings model.SiteSettings
+	var updatedAt string
+	err := s.db.QueryRow(
+		`SELECT site_name, site_description, signup_mode, updated_at
+		FROM site_settings WHERE id = 1`,
+	).Scan(&settings.SiteName, &settings.SiteDescription, &settings.SignupMode, &updatedAt)
+	if err != nil {
+		return nil, fmt.Errorf("query site settings: %w", err)
+	}
+
+	settings.UpdatedAt, _ = time.Parse(time.RFC3339, updatedAt)
+	return &settings, nil
+}
+
+// Update updates the site settings.
+func (s *SettingsStore) Update(siteName, siteDescription, signupMode string) error {
+	_, err := s.db.Exec(
+		`UPDATE site_settings SET site_name = ?, site_description = ?, signup_mode = ?,
+			updated_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now')
+		WHERE id = 1`,
+		siteName, siteDescription, signupMode,
+	)
+	return err
+}