Annotate un-annotated functions and arguments across src/ and demo.py

[tschm]

Subcategory: Linting/style + Type safety
Score: linting 4 → ~7, type safety 3 → ~5

This is the highest-leverage single change in the batch: the missing annotations are counted by both gates — 17 ANN* errors in make fmt and 13 no-untyped-def/no-untyped-call errors in mypy --strict are the same defect seen by two tools.

What to change

• src/pycharting/core/server.py:30 — NoCacheStaticFiles.get_response, arg scope
• src/pycharting/core/server.py:207 — health_check return type
• src/pycharting/core/server.py:213 — not_found_handler(request, exc) → (request: Request, exc: Exception) -> JSONResponse
• src/pycharting/core/server.py:220 — server_error_handler(request, exc) → same
• src/pycharting/core/lifecycle.py:237 — __enter__ → -> "ChartServer"
• src/pycharting/core/lifecycle.py:242 — __exit__, plus its three params (exc_type, exc_val, exc_tb)
• src/pycharting/api/routes.py:96 — get_data return type
• src/pycharting/data/ingestion.py:363 — slice_opt(arr)
• demo.py:45 — generate_ohlc

done when

ruff reports no ANN001/ANN201/ANN202/ANN204, and mypy --strict reports no no-untyped-def or no-untyped-call.

Evidence

ANN204 Missing return type annotation for special method `__enter__`
  --> src/pycharting/core/lifecycle.py:237:9

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Surfaced by the Rhiza quality gates during the template bump to jebel-quant/rhiza@v1.2.2, which enabled the stricter ruff rule families (ANN, A, BLE, ARG) and TYPECHECKER=both. Gate status at time of filing: make fmt FAIL, make typecheck FAIL, make docs-coverage FAIL, test-layout FAIL; make deptry, make security, make test PASS (154 tests, 100% coverage).