fix(coolify): strip is_build_time from env writes; add reveal + GCS

Coolify v4's POST/PATCH /applications/{uuid}/envs only accepts key,
value, is_preview, is_literal, is_multiline, is_shown_once. Sending
is_build_time triggers a 422 "This field is not allowed." — it's now
a derived read-only flag (is_buildtime) computed from Dockerfile ARG
usage. Breaks agents trying to upsert env vars.

Three-layer fix so this can't regress:
  - lib/coolify.ts: COOLIFY_ENV_WRITE_FIELDS whitelist enforced at the
    network boundary, regardless of caller shape
  - app/api/workspaces/[slug]/apps/[uuid]/envs: stops forwarding the
    field; returns a deprecation warning when callers send it; GET
    reads both is_buildtime and is_build_time for version parity
  - app/api/mcp/route.ts: same treatment in the MCP dispatcher;
    AI_CAPABILITIES.md doc corrected

Also bundles (not related to the above):
  - Workspace API keys are now revealable from settings. New
    key_encrypted column stores AES-256-GCM(VIBN_SECRETS_KEY, token).
    POST /api/workspaces/[slug]/keys/[keyId]/reveal returns plaintext
    for session principals only; API-key principals cannot reveal
    siblings. Legacy keys stay valid for auth but can't reveal.
  - P5.3 Object storage: lib/gcp/storage.ts + lib/workspace-gcs.ts
    idempotently provision a per-workspace GCS bucket, service
    account, IAM binding and HMAC key. New POST /api/workspaces/
    [slug]/storage/buckets endpoint. Migration script + smoke test
    included. Proven end-to-end against prod master-ai-484822.

Made-with: Cursor

scripts/migrate-api-keys-revealable.sql

@@ -0,0 +1,25 @@
+-- =============================================================================
+-- Make workspace API keys revealable.
+--
+-- Adds `key_encrypted` — base64 of secret-box(VIBN_SECRETS_KEY, plaintext token).
+-- Existing rows keep `key_encrypted = NULL` and are therefore NOT revealable;
+-- only the hash was stored at mint time and the plaintext is unrecoverable by
+-- design. Those keys still work for auth (hash lookup is unchanged); they just
+-- can't surface the plaintext again — the UI will flag them as legacy.
+--
+-- New keys minted after this migration will populate `key_encrypted` and can
+-- be revealed on demand by session-authenticated users (never by API-key
+-- principals — prevents lateral movement).
+--
+-- Safe to re-run.
+-- =============================================================================
+
+ALTER TABLE vibn_workspace_api_keys
+  ADD COLUMN IF NOT EXISTS key_encrypted TEXT;
+
+COMMENT ON COLUMN vibn_workspace_api_keys.key_encrypted IS
+  'base64( AES-256-GCM encrypt(VIBN_SECRETS_KEY, plaintext vibn_sk_...) ). '
+  'NULL for legacy rows minted before this column existed — those keys '
+  'remain valid for auth but cannot be revealed.';
+
+SELECT 'API-key revealability migration complete' AS status;