M1-T04: converge startup chain onto the single app DB

run_all_migrations() now adopts/initializes only the app DB and returns
{'app': ...}. app/main.py drops the location/poo readiness checks
(ensure_location_db_ready / ensure_poo_db_ready) and their imports;
ensure_runtime_dirs only provisions the app DB path; lifespan still
fail-closes on a missing/unmanaged app DB. Delete the retired
location/poo adopt scripts and the alembic_location / alembic_poo
chains. Update tests to single-DB expectations and drop the obsolete
location/poo adoption + readiness tests.

pytest 95 passed; ruff clean (pre-existing only); a fresh app DB
initialized via scripts.run_migrations contains location + poo_records.

scripts/run_migrations.py

@@ -2,16 +2,12 @@ from __future__ import annotations
 
 from app.config import get_settings
 from scripts.app_db_adopt import adopt_or_initialize_app_db
-from scripts.location_db_adopt import adopt_or_initialize_location_db
-from scripts.poo_db_adopt import adopt_or_initialize_poo_db
 
 
 def run_all_migrations() -> dict[str, str]:
     settings = get_settings()
     return {
         "app": adopt_or_initialize_app_db(settings.app_database_url),
-        "location": adopt_or_initialize_location_db(settings.location_database_url),
-        "poo": adopt_or_initialize_poo_db(settings.poo_database_url),
     }