feat(db): Optional PostgreSQL support for the main app database

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Summary

Odysseus stores all primary state in a single SQLite file (data/app.db). SQLite is
perfect as the zero-config single-host default, but it's a single-file, single-writer
database — multiple Odysseus instances/endpoints can't point at the same data and read
and edit it concurrently. This PR adds optional, opt-in PostgreSQL support for the
main app database, engaged only when the operator sets
DATABASE_URL=postgresql://…, so user data can be hosted in the cloud and shared
across instances. SQLite stays the zero-config default, byte-for-byte unchanged. It is
the DB-only subset of the closed #3763 (which had working Postgres code but was closed
for bundling i18n/UI), kept deliberately single-purpose.

What changed

• core/database.py — add one IS_SQLITE flag (engine.dialect.name == "sqlite")
  and gate init_db(): the ~45 legacy SQLite-only _migrate_*() upgraders (raw
  sqlite3/PRAGMA/ALTER) run only on SQLite — a fresh Postgres DB never had an old
  schema to patch. create_all() (which builds the full portable schema) and the three
  encryption-at-rest migrations run on both dialects.
• mcp_servers/email_server.py — read email accounts via the app engine/ORM so the
  one main-app.db raw read follows DATABASE_URL instead of opening SQLite directly.
• requirements-optional.txt — add opt-in psycopg2-binary (never mandatory).
• tests/test_postgres_compat.py (new) — dual-dialect round-trip test.
• tests/test_mcp_email_decode_header_spaces.py — update the account fixture to the
  new engine-based read path (it was tied to the old raw-sqlite3 path).
• .github/workflows/ci.yml — add an informational python-tests-postgres job.
• .env.example, docs/setup.md — document the opt-in and its v1 limits.

Deliberate v1 behaviour (called out so it isn't read as a bug)

• Fresh-Postgres-only. No SQLite→Postgres data-copy tool yet; switching starts from
  an empty database. Legacy backfill/seed migrations are skipped on Postgres (a fresh DB
  has no legacy rows to patch). A dedicated migration tool is the natural follow-up.
• The email side-DBs (scheduled_emails.db / email_cache.db) stay local SQLite.
• Chat full-text search falls back from FTS5 to LIKE on Postgres.
• No Alembic — create_all() covers fresh installs; versioned migrations can come later.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #4807

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs — this is not a duplicate (issue Optional PostgreSQL backend for the main app database (enables cloud-hosted, multi-instance shared data) #4807; prior art feat: pt-BR/es-ES i18n, cozy UI refinement layer, Postgres support #3763 is closed).
• This PR targets dev.
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in. (The one extra test file is updated only because this change alters the code path it exercises.)
• I actually ran the app (uvicorn app:app) and verified the change works end-to-end against a real Postgres database — see "How to Test".

How to Test

Verified end-to-end on real PostgreSQL (managed cloud Postgres, over the network):

• tests/test_postgres_compat.py against the live DB → 2 passed (SQLite + Postgres
  legs). Asserts that JSON, Boolean, EncryptedText (encrypt→decrypt), and
  Integer-autoincrement values all round-trip correctly on Postgres.
• Real app, full restart: booted with DATABASE_URL=postgresql://…, created a note
  via POST /api/notes, killed the process and booted a fresh one against the same
  Postgres, and GET /api/notes returned the same note (also confirmed the row directly
  in Postgres via SQL). Proves data genuinely persists in Postgres across restarts.
• Clean boot: no PRAGMA/sqlite3 warnings or errors on the Postgres boot —
  confirms the IS_SQLITE gate correctly skips the SQLite-only migrations.

SQLite (default, unchanged):

• Ran the affected DB + MCP-email test files → 59 passed, 1 skipped (the Postgres leg
  skips automatically without TEST_DATABASE_URL). Includes the new test's SQLite leg
  and the updated test_mcp_email_decode_header_spaces.py.

Reproduce the Postgres leg yourself (also runs in the new CI job):

1. docker run -e POSTGRES_PASSWORD=postgres -p 5432:5432 postgres:16
2. pip install -r requirements-optional.txt
3. TEST_DATABASE_URL=postgresql://postgres:postgres@localhost:5432/odysseus_test python -m pytest -q tests/test_postgres_compat.py