feat: crypto portfolio analytics (history chart, allocation, P&L, cost basis)

Adds the analytics half of the CoinMarketCap reference: a value-over-time
chart (GET /crypto/history, forward-filled from AssetValuation like Net
Worth's own series but scoped to crypto assets), an allocation breakdown by
coin, and stat tiles for all-time profit, total invested (cost basis), and
best/worst performer.

Deliberately no "24h" range — history is only as dense as the sync cadence
(button or once/day), so a 24h chart would be one or two points, not a
smooth intraday line. Cost basis / all-time profit / best-worst performer
aren't separate endpoints: they're trivial to derive client-side from
fields already in GET /crypto/holdings, so no redundant backend logic.

Allocation card follows the dataviz skill's categorical rule: top 7 coins
by value get a fixed-order color from the app's already-validated 8-slot
ramp, an 8th+ folds into "Other" rather than generating a new hue.

82/82 backend tests passing (3 new for /crypto/history). Verified live
against the real CoinGecko API — and caught a real holding the user had
already added directly through the UI between messages, which the smoke
test correctly left untouched while cleaning up only its own test data.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01855PiW5Wd8n4aRW4n9kByY

backend/app/schemas/crypto.py

@@ -93,3 +93,21 @@ class CryptoSearchResult(BaseModel):
     symbol: str
     name: str
     thumb_url: str | None
+
+
+class CryptoHistoryPoint(BaseModel):
+    date: date_
+    value: Decimal
+
+
+class CryptoHistoryResponse(BaseModel):
+    """Total crypto holdings value over time — same "cumulative point
+    events, forward-filled" shape as NetWorthSummary's own series, but
+    scoped to crypto-class assets only (see services/crypto_service.py's
+    get_crypto_history). Powers the History chart on the Crypto tab."""
+
+    range: str
+    current: Decimal
+    change_amount: Decimal
+    change_percent: float | None
+    series: list[CryptoHistoryPoint]