Implement FIN-WP-0001-T06 HTTP runway read surface

Add create_runway_router() following the hub-core factory pattern,
expose GET /runway/summary and /runway/monthly-burn via create_app,
and add finhub serve for local operation.

src/fin_hub/schemas/__init__.py

@@ -0,0 +1,5 @@
+"""Pydantic schemas for fin-hub HTTP surfaces."""
+
+from fin_hub.schemas.runway import MonthlyBurnRead, RunwayAlertRead, RunwaySummaryRead
+
+__all__ = ["MonthlyBurnRead", "RunwayAlertRead", "RunwaySummaryRead"]

src/fin_hub/schemas/runway.py

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+"""Read schemas for runway and monthly burn HTTP surfaces."""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field
+
+
+class MonthlyBurnRead(BaseModel):
+    period_month: str
+    currency: str
+    total: float
+    entry_count: int
+
+
+class RunwayProjectionRead(BaseModel):
+    current_balance: float
+    monthly_burn: float
+    months_remaining: float | None = Field(
+        description="Projected runway in months; null when burn is zero and balance is positive.",
+    )
+    months_remaining_label: str
+    alert_threshold_months: float
+    below_threshold: bool
+    currency: str
+    computed_at: str
+
+
+class RunwayAlertRead(BaseModel):
+    code: str
+    severity: str
+    summary: str
+    detail: dict
+
+
+class RunwaySummaryRead(BaseModel):
+    runway: RunwayProjectionRead
+    alerts: list[RunwayAlertRead]
+    opening_balance: float | None
+    opening_balance_currency: str
+    ledger_path: str