Replace Google Tasks service-account auth with real OAuth

Service-account auth structurally cannot see a regular user's personal
task lists (no equivalent of Calendar's per-item sharing model) --
confirmed via GetTaskLists returning exactly the service account's own
empty "My Tasks" list, never the real user's three lists. Zero rows
were ever cached in production as a result.

Adds a standard 3-legged OAuth flow: /settings/google-tasks/connect
redirects to Google's consent screen (AccessTypeOffline+ApprovalForce
so a refresh_token is always issued), /callback exchanges the code and
persists the token (new oauth_tokens table), /disconnect clears it.
GoogleTasksClient now takes an option.ClientOption instead of a
credentials file path; NewGoogleTasksOAuthClient wraps it with a
dbTokenSource that reloads/refreshes from the DB on each access-token
expiry and re-persists -- carefully preserving the original
refresh_token when Google's refresh response omits one (it usually
does), which would otherwise silently and permanently break future
refreshes.

Settings page shows connection status and a Connect/Disconnect
button. Calendar keeps using service-account auth (that one actually
works). Requires a one-time manual step: create an OAuth 2.0 Client ID
in Google Cloud Console and set GOOGLE_OAUTH_CLIENT_ID/SECRET in .env
-- documented in .env.example.

1 files changed, 62 insertions, 0 deletions

diff --git a/internal/store/oauth_tokens.go b/internal/store/oauth_tokens.go
new file mode 100644
index 0000000..12df515
--- /dev/null
+++ b/internal/store/oauth_tokens.go
@@ -0,0 +1,62 @@
+package store
+
+import (
+	"database/sql"
+	"time"
+
+	"golang.org/x/oauth2"
+)
+
+// GetOAuthToken returns the stored token for a source (e.g. "google_tasks"),
+// or nil if none has been saved yet (integration not connected).
+func (s *Store) GetOAuthToken(source string) (*oauth2.Token, error) {
+	var accessToken, refreshToken, tokenType string
+	var expiry sql.NullTime
+	err := s.db.QueryRow(`
+		SELECT access_token, refresh_token, token_type, expiry FROM oauth_tokens WHERE source = ?
+	`, source).Scan(&accessToken, &refreshToken, &tokenType, &expiry)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	tok := &oauth2.Token{
+		AccessToken:  accessToken,
+		RefreshToken: refreshToken,
+		TokenType:    tokenType,
+	}
+	if expiry.Valid {
+		tok.Expiry = expiry.Time
+	}
+	return tok, nil
+}
+
+// SaveOAuthToken upserts the token for a source. A refreshed token from
+// oauth2.TokenSource sometimes omits RefreshToken (Google only returns it
+// on the initial grant) -- callers must pass the original refresh token
+// through if the new one is empty, since ON CONFLICT here always
+// overwrites rather than preserving the old value.
+func (s *Store) SaveOAuthToken(source string, tok *oauth2.Token) error {
+	var expiry *time.Time
+	if !tok.Expiry.IsZero() {
+		expiry = &tok.Expiry
+	}
+	_, err := s.db.Exec(`
+		INSERT INTO oauth_tokens (source, access_token, refresh_token, token_type, expiry, updated_at)
+		VALUES (?, ?, ?, ?, ?, CURRENT_TIMESTAMP)
+		ON CONFLICT(source) DO UPDATE SET
+			access_token = excluded.access_token,
+			refresh_token = excluded.refresh_token,
+			token_type = excluded.token_type,
+			expiry = excluded.expiry,
+			updated_at = CURRENT_TIMESTAMP
+	`, source, tok.AccessToken, tok.RefreshToken, tok.TokenType, expiry)
+	return err
+}
+
+// DeleteOAuthToken removes a source's stored token (disconnect).
+func (s *Store) DeleteOAuthToken(source string) error {
+	_, err := s.db.Exec(`DELETE FROM oauth_tokens WHERE source = ?`, source)
+	return err
+}