Switch usage sync to Redis-only identities

[Willxup]

Summary

• Switch usage ingestion to the Redis-only path and remove legacy export/snapshot run code.
• Split Redis usage processing from metadata sync; add independent metadata runner, manual sync path, and focused app wiring.
• Add schema migrations for Redis usage fields, legacy snapshot cleanup, unified usage identities, stats backfill, legacy metadata table removal, and prefix-generated identity cleanup.
• Replace auth file/provider metadata tables and APIs with unified usage_identities, including aggregated usage stats and safe public identity responses.
• Update metadata sync to write usage identities only, preserve provider partial-failure behavior, and stop syncing provider prefix values as identities.
• Update Usage Events, Credentials, Analysis, and Pricing frontend/backend contracts for unified identities and stable source filters.
• Filter zero-request source options from usage event filters and remove obsolete time params from the filter endpoint/client.
• Fix credential source tagging, CPA metadata response decoding, mobile pricing card layout, and Overview cost availability for zero-token unpriced models.
• Update README wording and configuration examples for the Redis-only sync behavior.

Closes #19

Test plan

• go test ./cmd/... ./internal/...
• npm --prefix web test -- StatCards.test.ts CostTrendChart.test.ts
• npm --prefix web test -- api.test.ts
• npm --prefix web test -- UsagePage.logic.test.ts
• npm --prefix web run build
• git diff --check
• Started backend with project .env and manually smoke-tested UI/API