fix: align rest_proxy with schwab-py 1.5.1 enum API + pin dependency

[rdemeritt]

Root cause

requirements.txt left schwab-py unpinned, so the Docker build pulled a newer enum API than rest_proxy.py was written against. The installed version is schwab-py 1.5.1. Two live 502s resulted (greeksmith → proxy @ 127.0.0.1:8080):

• GET /v1/quotes?fields=quote → type object 'Quote' has no attribute 'FIELD_QUOTE'
• GET /v1/pricehistory?...frequency_type=daily&period=1 → expected type "Period", got type "int"

Audited all four /v1 endpoints; chains and markets were also wrong (chains used the dropped range= kwarg + an unmapped entitlement; markets relied on name-only enum lookup).

API corrections (old → new), validated against schwab-py 1.5.1

Endpoint	Old	New
quotes	client.Quote.FIELD_QUOTE / FIELD_REFERENCE / FIELD_EXTENDED / FIELD_FUNDAMENTAL / FIELD_REGULAR	client.Quote.Fields.QUOTE / .REFERENCE / .EXTENDED / .FUNDAMENTAL / .REGULAR
pricehistory	raw int for period, frequency	PriceHistory.Period(int), PriceHistory.Frequency(int) (typed enums; both int-valued)
pricehistory	period_type/frequency_type best-effort name lookup	typed PeriodType/FrequencyType (by value day/daily or by name)
chains	kwargs["range"] = ...	kwargs["strike_range"] = ... (get_option_chain renamed the kwarg; range= was rejected)
chains	entitlement passed through as raw str	mapped to Options.Entitlement
chains	Options.ContractType/Strategy/StrikeRange/ExpirationMonth/Type via [NAME] only	mapped via shared coercer (member name or wire value, e.g. ITM)
markets	MarketHours.Market[NAME] only	mapped via shared coercer (value equity or name EQUITY)

Fix design

• New app/schwab_data_proxy/enum_mapping.py: centralized, unit-testable mappers. Each accepts the live/mocked client (reads enum classes off it), accepts member name or wire value, and raises UnknownEnumValue on bad input.
• rest_proxy.py: all four endpoints rewired to the mappers. UnknownEnumValue now surfaces as HTTP 400 (bad query string) instead of an opaque 502.
• I kept enforce_enums at the schwab-py default (on) and passed correct typed enums — that's the intended usage for 1.5.1 and keeps bad params loud rather than silently shipping wrong types upstream.

Dependency pin

requirements.txt: schwab-py==1.5.1 (with rationale comment) so the enum API can't silently drift again.

Tests / CI

• New tests/ pytest suite (19 tests): param→enum mapping + handler-level call construction with a mocked client (no network/auth). Asserts correct enum types and kwarg names, and that bad params → 400.
• Tests run against the real installed schwab-py enums, so a future upgrade that renames/retypes a member fails CI loudly.
• New CI test job (pytest) + requirements-dev.txt + pyproject.toml pytest config.
• Local results: 19 passed; ruff check app/ + ruff format --check app/ clean.

Deploy note

This is a code-only change. The running proxy container must be rebuilt/restarted by the operator for the fix to take effect (so greeksmith's data smoke can pass). No container was rebuilt or deployed in this PR. Do not merge until reviewed.