feat: fb and google oauth

migrations/0012_create_oauth_accounts.down.sql

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+DROP TABLE IF EXISTS oauth_accounts;
+-- Note: re-imposing NOT NULL on users.password_hash would require backfilling
+-- any OAuth-only users with a dummy hash; left as a manual step.

migrations/0012_create_oauth_accounts.up.sql

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+-- OAuth account linkage.
+--
+-- One row per (provider, provider_user_id). A single user can have several
+-- providers linked (e.g. signed up with email + later linked Google). The
+-- provider's `sub` (Google) / `id` (Facebook) is the stable identity — email
+-- is captured for display but is NOT the join key, since providers can let
+-- users change the email on the account.
+--
+-- When a callback fires:
+--   1. Look up (provider, provider_user_id) — found → log that user in.
+--   2. Otherwise look up users.email — found → link new oauth_accounts row
+--      to the existing user and log them in.
+--   3. Otherwise create a new user (password_hash NULL) + an oauth_accounts
+--      row, log them in.
+CREATE TABLE IF NOT EXISTS oauth_accounts (
+    id                 UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id            UUID         NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    provider           VARCHAR(32)  NOT NULL,                -- "google" | "facebook"
+    provider_user_id   TEXT         NOT NULL,                -- sub (Google) / id (Facebook)
+    email              CITEXT,
+    name               TEXT,
+    avatar_url         TEXT,
+    created_at         TIMESTAMPTZ  NOT NULL DEFAULT now(),
+    updated_at         TIMESTAMPTZ  NOT NULL DEFAULT now(),
+    UNIQUE (provider, provider_user_id)
+);
+
+CREATE INDEX IF NOT EXISTS oauth_accounts_user_id_idx   ON oauth_accounts (user_id);
+CREATE INDEX IF NOT EXISTS oauth_accounts_provider_idx  ON oauth_accounts (provider);
+
+-- Password hash must be optional now: OAuth-only users have no password.
+-- Existing email/password users keep their hash; new OAuth users get NULL
+-- and can set a password later via the forgot-password flow if they want
+-- a fallback login method.
+ALTER TABLE users ALTER COLUMN password_hash DROP NOT NULL;