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/database/migrations/001_initial.sql

@@ -0,0 +1,72 @@
+-- Initial schema for Favoritter.
+-- Creates core tables: users, sessions, signup_requests, faves, tags, fave_tags, site_settings.
+
+CREATE TABLE users (
+    id                   INTEGER PRIMARY KEY,
+    username             TEXT    NOT NULL UNIQUE COLLATE NOCASE,
+    display_name         TEXT    NOT NULL DEFAULT '',
+    bio                  TEXT    NOT NULL DEFAULT '',
+    avatar_path          TEXT    NOT NULL DEFAULT '',
+    password_hash        TEXT    NOT NULL,
+    role                 TEXT    NOT NULL DEFAULT 'user' CHECK (role IN ('user', 'admin')),
+    profile_visibility   TEXT    NOT NULL DEFAULT 'public' CHECK (profile_visibility IN ('public', 'limited')),
+    default_fave_privacy TEXT    NOT NULL DEFAULT 'public' CHECK (default_fave_privacy IN ('public', 'private')),
+    must_reset_password  INTEGER NOT NULL DEFAULT 0,
+    disabled             INTEGER NOT NULL DEFAULT 0,
+    created_at           TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    updated_at           TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);
+
+CREATE TABLE sessions (
+    token      TEXT    PRIMARY KEY,
+    user_id    INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    expires_at TEXT    NOT NULL,
+    created_at TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);
+CREATE INDEX idx_sessions_user_id ON sessions(user_id);
+CREATE INDEX idx_sessions_expires ON sessions(expires_at);
+
+CREATE TABLE signup_requests (
+    id            INTEGER PRIMARY KEY,
+    username      TEXT    NOT NULL UNIQUE COLLATE NOCASE,
+    password_hash TEXT    NOT NULL,
+    status        TEXT    NOT NULL DEFAULT 'pending' CHECK (status IN ('pending', 'approved', 'rejected')),
+    created_at    TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    reviewed_at   TEXT,
+    reviewed_by   INTEGER REFERENCES users(id)
+);
+
+CREATE TABLE faves (
+    id          INTEGER PRIMARY KEY,
+    user_id     INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    description TEXT    NOT NULL,
+    url         TEXT    NOT NULL DEFAULT '',
+    image_path  TEXT    NOT NULL DEFAULT '',
+    privacy     TEXT    NOT NULL DEFAULT 'public' CHECK (privacy IN ('public', 'private')),
+    created_at  TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    updated_at  TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);
+CREATE INDEX idx_faves_user_id ON faves(user_id);
+CREATE INDEX idx_faves_privacy ON faves(privacy);
+CREATE INDEX idx_faves_created ON faves(created_at);
+
+CREATE TABLE tags (
+    id   INTEGER PRIMARY KEY,
+    name TEXT    NOT NULL UNIQUE COLLATE NOCASE
+);
+
+CREATE TABLE fave_tags (
+    fave_id INTEGER NOT NULL REFERENCES faves(id) ON DELETE CASCADE,
+    tag_id  INTEGER NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+    PRIMARY KEY (fave_id, tag_id)
+);
+CREATE INDEX idx_fave_tags_tag_id ON fave_tags(tag_id);
+
+CREATE TABLE site_settings (
+    id               INTEGER PRIMARY KEY CHECK (id = 1),
+    site_name        TEXT NOT NULL DEFAULT 'Favoritter',
+    site_description TEXT NOT NULL DEFAULT '',
+    signup_mode      TEXT NOT NULL DEFAULT 'open' CHECK (signup_mode IN ('open', 'requests', 'closed')),
+    updated_at       TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);
+INSERT INTO site_settings (id) VALUES (1);