feat(api): schematics catalog (backend + data model + 3 CC0 seed SVGs)

[ohgeeceee]

First PR of the schematics MVP series. Establishes the read-only catalog that follow-ups (PR #2: viewer, PR #3: DTC cross-link) will build on.

Schema:

• New 'schematics' table with slug, series, system, file_path, mime, license, tags, year range, etc. Idempotent migration via CREATE TABLE IF NOT EXISTS.

API:

• GET /api/schematics — list/filter (series, system, q substring).
• GET /api/schematics/ — single row, 404 if unknown, 503 if the asset file is missing on disk (distinct from not-in-catalog).

Files served by nginx from /static/schematics/.svg, never through Python (same pattern the rest of the API uses).

Seed (CC0, hand-drawn):

• e89-z4-rcd3302-power-can — RCD 3302 retrofit, power + K-CAN tap
• e90-cas3-pinout — CAS3/4 connector pin reference (e90)
• e60-n54-dme-power — N54 DME main-relay + power tree (e60)

Wiring:

• bootstrap_dtc.py seeds schematics alongside DTCs on every deploy.
• No new Python deps. Stdlib-only, matches the rest of the backend.

Tests:

• test_schematics.py — 18 unit tests (schema, list filters, slug lookup, mime→extension map, seed idempotency).
• test_app_schematics_endpoints.py — 8 HTTP integration tests against a live ThreadingHTTPServer.
• Full suite: 109/109 passing.

Out of scope (follow-ups): viewer UI, DTC cross-link, community upload pipeline, hotspot annotations, desktop bundling.

Protected paths: none touched (this PR is backend-only).

Closes: schematics-mvp#1 of the agreed 3-PR plan.

What this adds

For data contributions (community/*.toml)

• Vehicle: chassis + engine + year
• How verified:
• Loads cleanly (checked the Diagnostics tab — no warnings)
• Original / community-derived knowledge, not from ISTA or other
  proprietary software

For code contributions

• cargo fmt and cargo clippy are clean
• Tested against the Simulator (and, if possible, a real car — say which)
• No BMW proprietary data or trademarks added

[ecc-tools]

ECC bundle files are already tracked in this repository. Skipping generation of another bundle PR.