feat: add optional PostgreSQL + pgvector backend

[chrisdietr]

Summary

Adds PostgreSQL + pgvector as an optional backend for QMD.

SQLite remains the default. PostgreSQL is opt-in via:

• QMD_BACKEND=postgres
• QMD_POSTGRES_URL=postgresql://...

This version is rebuilt against the current v2 layout on main, so the diff matches the current code organization instead of the old pre-v2 structure.

Why

SQLite is still the best default for local single-user usage.

PostgreSQL is useful for shared or multi-process deployments where multiple agents or services need concurrent access to the same QMD index.

What changed

• backend selection in src/db.ts
• PostgreSQL adapter in src/pg.ts
• PostgreSQL worker in src/pg-worker.ts
• PostgreSQL schema/store support in src/store.ts
• CLI status output updated in src/cli/qmd.ts
• SDK wording updated in src/index.ts
• docs and changelog updates
• opt-in PostgreSQL integration tests

Out of scope

• automatic migration from SQLite to PostgreSQL
• changing the default backend
• MCP-specific feature changes beyond store/backend compatibility
• larger public SDK API redesign

Reviewer guide

Suggested review order:

1. src/db.ts
2. src/pg.ts
3. src/pg-worker.ts
4. src/store.ts
5. src/cli/qmd.ts
6. src/index.ts
7. test/store.postgres.test.ts
8. README.md
9. CHANGELOG.md

Validation

• bun run build
• bun test --preload ./src/test-preload.ts test/sdk.test.ts
• PostgreSQL integration tests are opt-in
• sqlite-vec-dependent tests still depend on local extension-loading support

AI-assisted development notes

Prompts used

• Port the PostgreSQL backend work onto the current v2 layout without reintroducing obsolete files.
• Move CLI-related changes from the old src/qmd.ts location into src/cli/qmd.ts.
• Keep SQLite as the default backend and make PostgreSQL opt-in.
• Avoid unnecessary MCP-specific branching if the store/SDK layer already handles backend differences.

Decision summary

• Rebuilt this as a v2-native change instead of preserving the old pre-v2 file layout.
• Kept backend selection environment-driven in this PR to limit surface-area changes.
• Left MCP mostly untouched because it already consumes the SDK/store layer.
• Added PostgreSQL coverage as opt-in integration tests.
• Updated SDK wording to be backend-neutral and exposed QMDStore.backend.

Alternatives considered

• Updating the original pre-v2 PR directly
  Rejected because the resulting diff was harder to review after the v2 refactor.

• Adding explicit PostgreSQL constructor options to the stable SDK in this PR
  Deferred to keep this change focused on backend support.

Validation performed

• verified PostgreSQL schema initialization path
• verified PostgreSQL FTS behavior in tests
• verified pgvector-backed similarity search path in tests
• verified CLI status output logic for SQLite vs PostgreSQL
• verified TypeScript build and SDK tests