feat(backend): add pricing coverage, unpriced seats, and explain endpoints

add backend endpoints for pricing coverage analysis and unpriced seat inspection

add explain endpoint to make effective pricing decisions traceable
improve pricing diagnostics for admin and editor workflows

backend/app/schemas/pricing_diagnostics.py

@@ -0,0 +1,52 @@
+from pydantic import BaseModel
+
+
+class PricingExplainMatchedRule(BaseModel):
+    matched_rule_level: str
+    matched_target_ref: str
+    pricing_category_id: str
+    amount: str
+    currency: str
+
+
+class PricingExplainResponse(BaseModel):
+    scheme_id: str
+    scheme_version_id: str
+    seat_id: str | None
+    element_id: str | None
+    sector_id: str | None
+    group_id: str | None
+    row_label: str | None
+    seat_number: str | None
+    has_price: bool
+    reason_code: str
+    reason_message: str
+    matched_rule: PricingExplainMatchedRule | None
+
+
+class UnpricedSeatItem(BaseModel):
+    seat_record_id: str
+    seat_id: str | None
+    element_id: str | None
+    sector_id: str | None
+    group_id: str | None
+    row_label: str | None
+    seat_number: str | None
+    reason_code: str
+    reason_message: str
+
+
+class UnpricedSeatsResponse(BaseModel):
+    scheme_id: str
+    scheme_version_id: str
+    total: int
+    items: list[UnpricedSeatItem]
+
+
+class PricingCoverageSummaryResponse(BaseModel):
+    scheme_id: str
+    scheme_version_id: str
+    total_seats: int
+    priced_seats: int
+    unpriced_seats: int
+    coverage_percent: float