docs: add ADR for ft-categories-on-off feature

[soraiayugulis]

Summary

Adds the full ADR for the ft-categories-on-off feature — specification and implementation plan — before any code is written.

Changes

docs/adr/ft-categories-on-off/specification.md

• 15 Facts in Given/When/Then format
• 7 Rules with deterministic input→output mappings
• 19 Test Cases (TC-001 to TC-019) mapped to Facts and Rules
• Error codes table with HTTP status
• Edge cases section
• Testing strategy using FakeFeatureFlagClient (offline, no ConfigCat SDK key required)
• Smoke test strategy for validating real SDK when key becomes available
• GitHub Packages authentication guide for local and CI environments
• All open questions resolved

docs/adr/ft-categories-on-off/implementation-plan.md

• 5 sequential phases with declared dependencies and dedicated branches
• Phase 0 — library integration, GitHub Packages auth, FakeFeatureFlagClient setup
• Phase 1 — CategoryStatus enum, Card extension with cascade on deactivate(), BalanceResponse extension
• Phase 2 — CATEGORY_INACTIVE authorization rule + MEAL fallback skip logic
• Phase 3 — PUT /cards/{cardId}/categories/{category}/status endpoint + card activation support
• Phase 4 — @WatchFeatureFlag and @HiFeatureFlag per-method AOP instrumentation
• Phase 5 — integration tests + ConfigCat smoke test
• Each sub-task includes target file, steps, test names, and commit message

Notes

• BalanceResponse record breaking change is documented with exact line references (CardController lines 47 and 62)
• Card.deactivate() extension (cascade) and new activate() method are scoped to Phase 1
• No implementation is included in this PR — authorization required after plan review

Checklist

• Facts and Rules are consistent with each other
• All Facts are covered by at least one Test Case
• Phase dependencies are correctly ordered
• BalanceResponse breaking change impact is understood