feat(env): persistent dev secrets via .env.secrets override

Local dev secrets like MANA_STT_API_KEY had no persistent home — they
lived only in the gitignored, generator-overwritten per-app .env files.
Every `pnpm setup:env` wiped them, so devs had to re-paste keys after
any env regeneration. Same recurring friction for MANA_LLM_API_KEY,
MANA_AUTH_KEK, OAuth keys, etc.

New layer: `.env.secrets` at the repo root.

- Gitignored, optional, never required for the build to pass
- Read by generate-env.mjs AFTER .env.development; non-empty values
  override the matching key, so the merged result drives every per-app
  .env the generator writes
- Empty values fall through to the .env.development defaults — a
  freshly-copied .env.secrets.example is a no-op
- One source of truth for all dev secrets, propagated to every app
  with one `pnpm setup:env`

Files:
- `.env.secrets.example` — committed template documenting all known
  secret keys (mana-stt, mana-llm, auth KEK, sync JWT, MinIO, third-
  party APIs). Devs `cp .env.secrets.example .env.secrets` and fill in.
- `.gitignore` — ignores .env.secrets, allows .env.secrets.example
- `scripts/generate-env.mjs` — loads .env.secrets if present, prints
  "Loaded N secrets from .env.secrets" so devs see the override
  taking effect
- `scripts/setup-secrets.mjs` + `pnpm setup:secrets` — convenience
  script that SSHes to mana-server, greps the prod .env for the keys
  defined in .env.secrets.example, and writes them locally. Confirms
  before overwriting an existing .env.secrets unless --force is set;
  reports which keys couldn't be found on the remote so devs know
  what's left to fill manually
- `docs/LOCAL_DEVELOPMENT.md` + `docs/ENVIRONMENT_VARIABLES.md` —
  walk-through and architecture diagram update

Verified end-to-end:
- `rm .env.secrets apps/mana/apps/web/.env && pnpm setup:env` →
  STT key empty (no regression for devs who haven't opted in)
- `pnpm setup:secrets --force && pnpm setup:env` →
  STT key propagated, "Loaded 3 secrets from .env.secrets" in output
- POST /api/v1/voice/transcribe with a real audio file →
  full transcript back via gpu-stt.mana.how, end-to-end working

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/ENVIRONMENT_VARIABLES.md

@@ -17,16 +17,26 @@ That's it! All app-specific `.env` files are generated from `.env.development`.
 ## How It Works
 
 ```
-.env.development          # Central config (committed)
+.env.development          # Central config (committed, no secrets)
+        │
+        ├── .env.secrets   # Optional gitignored override (your API keys)
+        ▼
+scripts/generate-env.mjs   # Merges + transforms variables
         │
         ▼
-scripts/generate-env.mjs  # Transforms variables
-        │
-        ▼
-apps/**/apps/**/.env      # Generated files (gitignored)
+apps/**/apps/**/.env       # Generated files (gitignored)
 ```
 
-The generator reads `.env.development` and creates app-specific `.env` files with the correct prefixes for each platform:
+The generator reads `.env.development` first, then layers `.env.secrets` (if it exists) on
+top — non-empty values in `.env.secrets` override the matching key in `.env.development`.
+This is where personal dev secrets like `MANA_STT_API_KEY` live, so you don't have to
+re-paste them into per-app `.env` files after every `pnpm setup:env`.
+
+To populate `.env.secrets` from the Mac Mini in one shot, run `pnpm setup:secrets` (see
+[`docs/LOCAL_DEVELOPMENT.md`](LOCAL_DEVELOPMENT.md#personal-dev-secrets--envsecrets) for
+the full walk-through).
+
+The generator then creates app-specific `.env` files with the correct prefixes for each platform:
 
 | Platform | Prefix | Example |
 |----------|--------|---------|