feat: integrate CREEM web payment for credits purchase

Replace abandoned iOS App Store route with CREEM Merchant of Record
payment integration for web-based credits purchase.

Backend changes:
- Add CreemClient for CREEM API communication
- Add CreemService for checkout creation and webhook handling
- Add creem_transactions table for payment tracking
- Fix webhook payload parsing (id, order.id, customer.id structure)
- Integrate with existing points ledger system

Frontend changes:
- Display dynamic prices from CREEM API
- Support decimal price formatting (e.g., $1.00)
- Add checkout flow with redirect to CREEM hosted page

backend/src/core/config/settings.py

@@ -232,6 +232,13 @@ class AppleIapSettings(BaseModel):
     server_api_private_key: SecretStr | None = None
 
 
+class CreemSettings(BaseModel):
+    api_key: SecretStr | None = None
+    webhook_secret: SecretStr | None = None
+    base_url: str = "https://test-api.creem.io"
+    success_url: str = ""
+
+
 def _resolve_env_files() -> list[str]:
     """Resolve env files in order: .env.local overrides .env"""
     current = Path(__file__).resolve()
@@ -280,6 +287,7 @@ class Settings(BaseSettings):
     agent_runtime: AgentRuntimeSettings = Field(default_factory=AgentRuntimeSettings)
     points_policy: PointsPolicySettings = Field(default_factory=PointsPolicySettings)
     apple_iap: AppleIapSettings = Field(default_factory=AppleIapSettings)
+    creem: CreemSettings = Field(default_factory=CreemSettings)
     feedback_report: FeedbackReportSettings = Field(
         default_factory=FeedbackReportSettings
     )