M7-T01: add meter table + model + energy_cost_period.meter_id + backfill migration

app/models/energy.py

@@ -1,6 +1,7 @@
 """SQLAlchemy models for the energy pricing and DSMR metering subsystem.
 
-Five tables:
+Six tables:
+  - meter:                    physical electricity meter lifecycle epoch.
   - dsmr_reading:             raw DSMR telegram blobs (10-second down-sampled).
   - energy_contract:          contract head (manual or tibber, one active at a time).
   - energy_contract_version:  versioned pricing values; append-only for auditability.
@@ -19,6 +20,62 @@ from sqlalchemy.types import JSON
 from app.db import Base
 
 
+class Meter(Base):
+    """One physical electricity meter's installation epoch.
+
+    A ``meter`` record represents the period ``[started_at, ended_at)`` during
+    which a particular physical meter was installed and active.  Replacing a meter
+    (swap, home move, etc.) is modelled by closing the current record
+    (``ended_at = swap_timestamp``) and opening a new one
+    (``started_at = swap_timestamp``).
+
+    **Invariant**: for each ``commodity`` there is at most one active meter
+    (``ended_at IS NULL``) at any point in time.  The service layer enforces
+    this — no DB-level constraint is added to keep the migration simple and to
+    allow the application to return a meaningful error message.
+
+    ``commodity`` defaults to ``"electricity"``; the field is a free-form string
+    (no CHECK constraint) so future commodities (``gas``, ``heating``) can be
+    added without a schema change.
+
+    ``reason`` captures why this epoch started — one of ``initial``,
+    ``meter_swap``, ``home_move``, or ``other`` — stored as a plain string so
+    the application layer controls the allowed set.
+    """
+
+    __tablename__ = "meter"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+
+    # Human-readable label for this physical meter (e.g. address, serial, tariff zone).
+    label: Mapped[str] = mapped_column(String(255), nullable=False)
+
+    # Energy commodity this meter measures.  Defaults to "electricity".
+    commodity: Mapped[str] = mapped_column(String(32), nullable=False, default="electricity")
+
+    # UTC timestamp when this meter epoch starts (inclusive).  May be in the past
+    # (retroactive declaration); effective billing start = max(started_at, data start).
+    started_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+
+    # UTC timestamp when this meter epoch ends (exclusive).  NULL = currently active.
+    ended_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+
+    # Why this epoch was created.  Application-layer validation enforces the
+    # allowed set; no CHECK constraint to keep migrations simple.
+    reason: Mapped[str] = mapped_column(String(64), nullable=False)
+
+    # Free-form note (e.g. location, physical meter id, reason details).
+    note: Mapped[str | None] = mapped_column(String(1024), nullable=True)
+
+    # UTC timestamp of when this row was created.
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+
+    # Relationship to cost periods attributed to this meter epoch (not loaded eagerly).
+    cost_periods: Mapped[list["EnergyCostPeriod"]] = relationship(
+        back_populates="meter", cascade="save-update, merge"
+    )
+
+
 class DsmrReading(Base):
     """One down-sampled DSMR telegram stored as a full JSON blob.
 
@@ -217,6 +274,14 @@ class EnergyCostPeriod(Base):
         ForeignKey("energy_contract_version.id", ondelete="RESTRICT"), nullable=True
     )
 
+    # FK to the meter epoch this period belongs to.  RESTRICT prevents deletion of
+    # a meter that still has attributed cost periods.  Nullable for backwards
+    # compatibility (pre-M7 rows) and degraded periods where the meter was not
+    # determinable.
+    meter_id: Mapped[int | None] = mapped_column(
+        ForeignKey("meter.id", ondelete="RESTRICT"), nullable=True
+    )
+
     # True when the period was computed with incomplete data (missing readings or
     # missing price); serves as a flag for later recomputation.
     degraded: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
@@ -229,6 +294,9 @@ class EnergyCostPeriod(Base):
         back_populates="cost_periods"
     )
 
+    # Relationship back to the meter epoch.
+    meter: Mapped["Meter | None"] = relationship(back_populates="cost_periods")
+
 
 # Index on recorded_at for efficient time-range queries on DSMR readings.
 # (The ORM-level index=True on recorded_at already creates ix_dsmr_reading_recorded_at;