Add optional Authentik (OIDC) sign-in with server-side preference sync

Signed-in users get their preferences (hidden feeds, weather location,
weather toggle) stored server-side keyed by their OIDC subject and synced
across devices. Anonymous visitors keep using browser localStorage, so the
site stays public. First sign-in migrates existing localStorage prefs up.

- config: [web.auth] section (issuer, client_id/secret, redirect, session_secret)
- storage: user_preferences table + Get/PutUserPrefs
- web/auth: OIDC code flow, HMAC-signed session cookie, CSRF state + nonce
- web/prefs_api: GET/PUT /api/preferences (auth-gated, 64KB cap)
- frontend: prefs.js sync layer seeds localStorage from server, pushes on write
- header: sign-in / account control

OIDC discovery is non-fatal at boot: if Authentik is down, Pete serves
anonymously rather than refusing to start.

internal/storage/schema.go

@@ -45,6 +45,14 @@ CREATE TABLE IF NOT EXISTS reactions (
     reacted_at INTEGER NOT NULL
 );
 
+CREATE TABLE IF NOT EXISTS user_preferences (
+    user_sub   TEXT PRIMARY KEY,
+    prefs      TEXT NOT NULL,
+    username   TEXT,
+    email      TEXT,
+    updated_at INTEGER NOT NULL
+);
+
 CREATE UNIQUE INDEX IF NOT EXISTS idx_post_log_guid_channel ON post_log(guid, channel);
 CREATE INDEX IF NOT EXISTS idx_post_log_event_id ON post_log(event_id);
 CREATE INDEX IF NOT EXISTS idx_post_log_channel_posted ON post_log(channel, posted_at);