fix: guard all legacy migrations against fresh schema for CI compat

On fresh databases, 0001_init.up.sql creates providers/provider_id
(not llm_providers/llm_provider_id). Migrations 0013, 0041, 0046, 0047
referenced the old names without guards, causing CI migration failures.

- 0013: check llm_provider_id column exists before adding old constraint
- 0041: check llm_providers table exists before backfill/constraint DDL
- 0046: wrap CREATE TABLE in DO block with llm_providers existence check
- 0047: use ALTER TABLE IF EXISTS + DO block guard

internal/db/sqlc/models.go

@@ -298,20 +298,6 @@ type LifecycleEvent struct {
 	CreatedAt   pgtype.Timestamptz `json:"created_at"`
 }
 
-type LlmProviderOauthToken struct {
-	ID               pgtype.UUID        `json:"id"`
-	LlmProviderID    pgtype.UUID        `json:"llm_provider_id"`
-	AccessToken      string             `json:"access_token"`
-	RefreshToken     string             `json:"refresh_token"`
-	ExpiresAt        pgtype.Timestamptz `json:"expires_at"`
-	Scope            string             `json:"scope"`
-	TokenType        string             `json:"token_type"`
-	State            string             `json:"state"`
-	PkceCodeVerifier string             `json:"pkce_code_verifier"`
-	CreatedAt        pgtype.Timestamptz `json:"created_at"`
-	UpdatedAt        pgtype.Timestamptz `json:"updated_at"`
-}
-
 type McpConnection struct {
 	ID            pgtype.UUID        `json:"id"`
 	BotID         pgtype.UUID        `json:"bot_id"`