chore: prepare release 4.2.0

docs/releases/v4.2.0.md

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+# MTG Python Deckbuilder v4.2.0
+
+## Summary
+Budget Mode: full price-aware deck building with live budget tracking, post-build review, pickup suggestions, and price charts. Also includes multi-copy conflict dialogs, RandomService infrastructure, and stale price warnings.
+
+## Added
+
+### Budget Mode
+- Budget configuration in the New Deck modal: total cap, optional per-card ceiling, soft/hard enforcement mode, and pool tolerance
+- Card prices shown throughout the build pipeline (list and thumbnail views) with a running budget counter chip
+- Over-budget cards highlighted with a yellow/gold border; basic lands excluded from all calculations
+- Budget summary bar in deck summary view; budget badge and over-budget panel on saved deck view
+- Post-build budget review panel (triggered when total exceeds cap by >10%): lists over-budget cards with up to 3 cheaper alternatives, each with a live Swap button
+- "Accept deck as-is" button in soft mode to bypass the review
+- Pickups list page (`/decks/pickups?name=`) — affordable cards you don't own yet, sorted by priority tier
+- Pool budget filter: cards over the per-card ceiling by more than the pool tolerance are excluded before building; configurable per build (default 15%)
+- Controlled by `ENABLE_BUDGET_MODE` environment variable (default: enabled)
+
+### Price Cache Infrastructure
+- `price` and `price_updated` columns added to parquet card database via `refresh_prices_parquet()`
+- Setup page Refresh button downloads fresh Scryfall bulk data before rebuilding the cache
+- Optional auto-refresh (`PRICE_AUTO_REFRESH=1`) and lazy per-card refresh (`PRICE_LAZY_REFRESH=1`, default enabled)
+- `POST /api/price/refresh` manual rebuild endpoint
+- "Card Price Cache Status" section on the Setup page with last-updated date and Refresh button
+- Footer shows price data date alongside Scryfall attribution
+- `build-similarity-cache` CI workflow now commits `prices_cache.json` and `prices_cache.json.ts` alongside the parquet
+
+### Price Charts
+- Donut/bar chart showing deck spend by card role (9 categories: Land, Ramp, Creature, Card Draw, Removal, Wipe, Protection, Synergy, Other)
+- Price histogram showing card count distribution across cost buckets
+- Basic lands excluded from all chart calculations
+
+### Stale Price Warnings
+- Cards with price data older than 24 hours flagged with a clock indicator (⏱) on card tiles, hover popup, budget review panel, and Pickups page
+- If >50% of deck prices are stale, a single banner replaces per-card indicators
+- Controlled by `PRICE_STALE_WARNING_HOURS` (default: 24; set to 0 to disable)
+
+### Multi-copy Dialogs
+- Include conflict dialog: typing a multi-copy archetype card (e.g., Hare Apparent) in Must Include now prompts for copy count with optional Thrumming Stone checkbox
+- Exclude conflict dialog: conflict popup when the same card appears in both the Multi-Copy Package selector and Must Exclude
+
+### RandomService & Diagnostics
+- `RandomService` wrapping seeded RNG operations with input validation
+- `InvalidSeedError` exception for seed validation failures
+- `GET /api/random/diagnostics` endpoint behind `WEB_RANDOM_DIAGNOSTICS=1` flag
+- Random Mode documentation in `docs/random_mode/`
+
+## Changed
+- **Build Deck button**: "Create" renamed to "Build Deck" in the New Deck modal for consistency with "Quick Build"
+
+## Fixed
+- **Stale price banner after refresh**: Refreshing the price cache on the Setup page now correctly clears the stale warning
+- **Multi-copy include count**: Archetype card count is now correctly applied when confirmed from the conflict dialog