M6-T04: add EnergyContract CRUD + versions + profile validation API

- app/services/contracts.py: create_contract, add_version (append-only,
  auto-closes prior version), activate_contract (mutex), and
  active_contract_version_at (half-open [from, to)) for the billing engine.
- app/schemas/energy_contract.py + routes/api/energy_contracts.py: 6 endpoints
  (GET/POST contracts, GET/PATCH contracts/{id}, POST .../versions, GET profiles).
  Values validated against pricing profiles (422 on mismatch); no DELETE.
- main.py registers router; OpenAPI re-exported; tests for CRUD/version/activate.

app/schemas/energy_contract.py

@@ -0,0 +1,148 @@
+"""Pydantic schemas for the EnergyContract CRUD + versioning API (M6-T04).
+
+Schema hierarchy
+----------------
+ContractVersionResponse   — single version row (id, dates, values, created_at)
+ContractResponse          — contract head (id, name, kind, active, currency, timestamps)
+ContractDetailResponse    — contract head + embedded versions list (for GET{id})
+ContractListResponse      — paginated list of ContractResponse items
+ContractCreate            — POST /api/energy/contracts body
+ContractPatch             — PATCH /api/energy/contracts/{id} body (all fields optional)
+VersionCreate             — POST /api/energy/contracts/{id}/versions body
+ProfilesResponse          — GET /api/energy/profiles response
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+from typing import Any
+
+from pydantic import BaseModel, Field
+
+
+# ---------------------------------------------------------------------------
+# Version schemas
+# ---------------------------------------------------------------------------
+
+
+class ContractVersionResponse(BaseModel):
+    """Response schema for a single EnergyContractVersion row."""
+
+    id: int
+    effective_from: datetime
+    effective_to: datetime | None
+    values: dict[str, Any]
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+# ---------------------------------------------------------------------------
+# Contract schemas
+# ---------------------------------------------------------------------------
+
+
+class ContractResponse(BaseModel):
+    """Response schema for a single EnergyContract (without embedded versions).
+
+    Used for list responses where embedding all versions would be expensive.
+    """
+
+    id: int
+    name: str
+    kind: str
+    active: bool
+    currency: str
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class ContractDetailResponse(BaseModel):
+    """Response schema for a single EnergyContract with full version history.
+
+    Returned by GET /api/energy/contracts/{id} and by successful POST / PATCH
+    operations where the caller needs to see all version data.
+    """
+
+    id: int
+    name: str
+    kind: str
+    active: bool
+    currency: str
+    created_at: datetime
+    updated_at: datetime
+    versions: list[ContractVersionResponse]
+
+    model_config = {"from_attributes": True}
+
+
+class ContractListResponse(BaseModel):
+    """Response schema for GET /api/energy/contracts."""
+
+    items: list[ContractResponse]
+    total: int
+
+
+# ---------------------------------------------------------------------------
+# Request schemas
+# ---------------------------------------------------------------------------
+
+
+class ContractCreate(BaseModel):
+    """Request body for POST /api/energy/contracts.
+
+    ``effective_from`` defaults to the current UTC time if not provided,
+    giving the first version an open-ended start from "now".
+    ``kind`` is validated at the application layer against the profile registry;
+    clients should send ``"manual"`` or ``"tibber"``.
+    """
+
+    name: str = Field(..., min_length=1, max_length=255)
+    kind: str = Field(..., min_length=1, max_length=32)
+    currency: str = Field(default="EUR", min_length=1, max_length=8)
+    values: dict[str, Any]
+    effective_from: datetime | None = Field(
+        default=None,
+        description=(
+            "UTC datetime from which the first pricing version is effective. "
+            "Defaults to the current UTC time when omitted."
+        ),
+    )
+
+
+class ContractPatch(BaseModel):
+    """Request body for PATCH /api/energy/contracts/{id}.
+
+    All fields are optional.  Sending ``active=true`` activates this contract
+    (deactivating all others); ``active=false`` deactivates it without affecting
+    other contracts.
+    """
+
+    name: str | None = Field(default=None, min_length=1, max_length=255)
+    active: bool | None = None
+
+
+class VersionCreate(BaseModel):
+    """Request body for POST /api/energy/contracts/{id}/versions."""
+
+    effective_from: datetime
+    values: dict[str, Any]
+
+
+# ---------------------------------------------------------------------------
+# Profile response schemas
+# ---------------------------------------------------------------------------
+
+
+class ProfilesResponse(BaseModel):
+    """Response schema for GET /api/energy/profiles.
+
+    ``profiles`` is a list of raw profile dicts as produced by
+    ``list_profiles()`` (Pydantic model dumps).  Each entry contains at minimum
+    ``kind`` and ``label``; the full nested structure allows the front-end to
+    render a type-appropriate form for each pricing profile.
+    """
+
+    profiles: list[dict[str, Any]]