hotfix: stop per-request OOM on location export endpoints

[jirhiker]

Problem

App Engine terminated the process mid-request: "the process that handled this request was found to be using too much memory and was terminated." Already on F4 (1GB), 1 worker — so it's a per-request memory spike, not an undersized instance.

Two endpoints loaded the entire SampleLocation table into RAM on a single request:

• GET /location/feature_collection — .all() the whole table, then built a second list of dicts (double memory).
• GET /location/shapefile — .all() the whole table and wrote locations.shp/.zip to CWD. On App Engine the app dir is read-only; only /tmp is writable and it's RAM-backed (counts against instance memory).

Both were async def doing blocking sync DB/file work, stalling the event loop.

Fix

api/base.py

• feature_collection → StreamingResponse, emits GeoJSON row-by-row via yield_per(1000). Whole table never buffered. Output shape unchanged.
• shapefile → writes to tempfile.mkdtemp() (not read-only CWD), streams rows with yield_per, cleans up the temp dir via BackgroundTask after send.
• Both async def → def (blocking work runs in threadpool).

db/__init__.py

• Production async engine echo=True → False (was logging every statement).
• Bounded pools on both engines: pool_size=5, max_overflow=2, pool_recycle=1800, pool_pre_ping=True.

Not in this PR (ops / follow-up)

• app.yaml entrypoint updated locally with --max-requests 200 --max-requests-jitter 40 --timeout 120 (worker recycling as a residual-leak safety net). Not committed — app.yaml is untracked and contains secrets.
• ⚠️ app.yaml holds a plaintext DB password + GCP service-account private key. Move to Secret Manager and rotate those credentials.

Testing

• Syntax verified. Not load-tested here — verify a feature_collection / shapefile call against the full table post-deploy.

🤖 Generated with Claude Code

[jirhiker]

Superseded by #762 — clean hotfix cherry-picked onto production (production has a refactored codebase; the original branch targeted the geoserver-iac line and produced a 100-commit diff against production).