fix(picloud): F-P-003 PICLOUD_DB_MAX_CONNECTIONS env knob, default 32 (matches gate)

init_db hard-coded max_connections(10). The execution gate
(ExecutionGate, PICLOUD_MAX_CONCURRENT_EXECUTIONS default 32) lets 32
script executions run concurrently, each doing multiple sequential
DB calls — plus the dispatcher tick every 100ms, the cron tick, three
GC sweeps, and the auth middleware running per request all draw from
the same pool. With max=10 vs gate=32, pool starvation surfaces as
acquire_timeout errors and tail-latency spikes under load.

- DEFAULT_DB_MAX_CONNECTIONS = 32 (matches gate default).
- Env knob: PICLOUD_DB_MAX_CONNECTIONS overrides; invalid/zero → default.
- Documented in CLAUDE.md runtime config table.

AUDIT.md anchor: F-P-003.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -116,6 +116,7 @@ Environment variables consumed by the `picloud` binary:
 | `PICLOUD_BIND` | `0.0.0.0:8080` | HTTP listen address. Port 8080 is owned by another process on this host — override locally. |
 | `PICLOUD_MAX_CONCURRENT_EXECUTIONS` | `32` | Global concurrency cap on data-plane script executions. Overflow returns HTTP 503 with `Retry-After: 1` immediately (no queue). |
 | `DATABASE_URL` | — | Required. Postgres connection string. |
+| `PICLOUD_DB_MAX_CONNECTIONS` | `32` | Postgres pool size. Matched to `PICLOUD_MAX_CONCURRENT_EXECUTIONS` so the data plane can't starve background workers. |
 | `PICLOUD_SESSION_TTL_HOURS` | `24` | Sliding-window session lifetime. |
 | `PICLOUD_SANDBOX_MAX_*` | conservative defaults | Per-knob admin ceilings on Rhai sandbox overrides. See `manager-core::sandbox::SandboxCeiling`. |
 | `PICLOUD_FILES_ROOT` | `./data` | Filesystem root for `files::*` blob storage (v1.1.5). Bytes live at `<root>/files/<app_id>/<collection>/<id[0:2]>/<id>`; metadata in Postgres. |