Phase 2: DATABASE_BACKEND switch, component config, schema scoping

- config: add DatabaseBackend enum + DATABASE_* component vars on
  CommonSettings (host/port/name/schema/user/password, defaults
  'meshcorehub'); single effective_database_url resolver with precedence
  DATABASE_URL > postgres (assembled, fail-fast on missing vars) > SQLite
  default. effective_database_schema returns the schema only for Postgres.
  Collapses the duplicated resolver/field out of Collector/API settings.
- database: create_database_engine/DatabaseManager accept a schema arg and
  scope Postgres connections via search_path (psycopg2 -c options; asyncpg
  server_settings). No-op for SQLite.
- alembic/env: migrate into the instance schema (version_table_schema +
  CREATE SCHEMA + search_path) for Postgres; fix online render_as_batch to
  be SQLite-only (matching the offline path).
- .env.example: document the DATABASE_* block.

SQLite behaviour unchanged: default no-env path resolves to the same URL
(new regression tests), full suite green (1051 passed), fresh db upgrade OK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -66,6 +66,29 @@ LOG_LEVEL=INFO
 #       └── meshcore.db    # SQLite database
 DATA_HOME=./data
 
+# -----------------------------------------------------------------------------
+# Database
+# -----------------------------------------------------------------------------
+# SQLite is the zero-config default and needs nothing here — it lives at
+# ${DATA_HOME}/collector/meshcore.db.
+#
+# To use PostgreSQL instead, set DATABASE_BACKEND=postgres and fill in the
+# DATABASE_* values below (the bundled postgres container derives its
+# POSTGRES_USER/PASSWORD/DB from DATABASE_USER/PASSWORD/NAME). You must also
+# activate the compose 'postgres' profile, e.g. `docker compose --profile postgres up`.
+#
+# DATABASE_BACKEND=postgres
+# DATABASE_HOST=postgres
+# DATABASE_PORT=5432
+# DATABASE_NAME=meshcorehub
+# DATABASE_SCHEMA=meshcorehub   # override per instance (e.g. prod, stg) on a shared cluster
+# DATABASE_USER=meshcorehub
+# DATABASE_PASSWORD=            # required for postgres; e.g. `openssl rand -base64 32`
+#
+# Advanced: set DATABASE_URL to a full SQLAlchemy URL to override all of the above
+# (e.g. a managed/external Postgres). Takes precedence over DATABASE_BACKEND.
+# DATABASE_URL=postgresql+psycopg2://user:pass@host:5432/dbname
+
 # Directory containing seed data files for import
 # Default: ./seed (relative to docker-compose.yml location)
 # Inside containers this is mapped to /seed