feat(infra): add partha:up instant restart and partha:down (#223 follow-up)

[parthrohit22]

Summary

Make the npm run partha one-command launcher convenient for both first-time users
and repeated local restarts. The default npm run partha keeps building images from
source (so any fresh clone works with no pre-built image — the "all users" property).
Add npm run partha:up for instant restarts that skip the build step and reuse the
already-built images, and npm run partha:down to stop the stack from another terminal.

This is dev-tooling/onboarding only. It introduces no new product surface, changes no
API contract, and preserves the existing security posture (Postgres/Redis host-isolated,
named volumes persist).

Linked issue

Part of #223 (full-stack compose launcher). Convenience follow-up requested during
verification of the re-run build behavior.

What changed

• Backend/infra (scripts/partha.mjs): accept a --no-build flag; when set, run
  docker compose up --wait instead of docker compose up --build --wait. All other
  behavior (Docker/buildx preflight, readiness waits, Ctrl+C → compose down) unchanged.
• Root package.json: add partha:up (node scripts/partha.mjs --no-build) and
  partha:down (docker compose down).
• README.md: document partha:up / partha:down and clarify that images are
  reused from cache on repeat runs.

Acceptance criteria completed

• npm run partha (with --build) still boots all four services and prints
  PARTHA is ready at http://localhost:5173; curl localhost:8000/ready → 200 and
  curl localhost:5173/ → 200.
• npm run partha:up starts the existing images with zero rebuild (Building lines
  = 0) and reaches ready; API + frontend return 200.
• npm run partha:down stops the stack cleanly.
• No change to OpenAPI contract, security boundaries, or volume persistence.
• docker compose -f docker-compose.yml config remains valid.

Testing performed

On macOS with Docker (colima) running:

• npm run partha: booted all 4 services, ready line printed, /ready 200, frontend 200.
• npm run partha:up: booted with 0 Building lines (image cache reused), ready line,
  /ready 200, frontend 200.
• npm run partha:down: containers stopped, no orphans; named volumes preserved.

Screenshots

Not applicable (infra/launcher script change; verified via curl + compose status).

Security and data considerations

None new. Postgres/Redis remain host-isolated (no host port bindings). No secrets added
to code or Compose. Named volumes (partha_postgres_data, partha_partha_storage)
preserve data across restarts as before.

Dependencies and blocked work

None. Depends on #223 (merged) for the base launcher.

Scope changes or remaining work

None. This is a convenience enhancement only.

Contributor checklist

• Branch named per CONTRIBUTING convention (feat/222-launcher-no-build)
• PR template fully filled
• Roadmap alignment stated (W7 deployment repeatability / §28)
• No new product surface; dev-tooling only
• Verification evidence included (real boot + curl)
• No secrets committed