feat: FASE 1 - Backend Core (modelli, auth, API)

Implementazione completa del backend FastAPI:
- Modelli SQLAlchemy: User, Recipe, RecipeVersion, RecipeTask,
  RecipeSubtask, Measurement, AccessLog, SystemSetting, RecipeVersionAudit
- Schemas Pydantic v2 per tutti i CRUD + statistiche SPC
- Middleware: API Key auth (X-API-Key) con role checking + access logging
- Router: auth, users, recipes, tasks, measurements, files, settings
- Services: auth (bcrypt+secrets), recipe (copy-on-write versioning),
  measurement (auto pass/fail con UTL/UWL/LWL/LTL)
- Alembic env.py con import modelli attivi
- Fix architect review: no double-commit, recipe_id subquery filter,
  user_id in access logs, type annotations corrette

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/models/measurement.py

@@ -0,0 +1,55 @@
+"""Measurement model for storing individual measurements."""
+from datetime import datetime
+from typing import Optional
+
+from sqlalchemy import (
+    BigInteger, Boolean, DECIMAL, DateTime, Enum, ForeignKey,
+    Integer, String, func,
+)
+from sqlalchemy.orm import Mapped, mapped_column
+
+from database import Base
+
+
+class Measurement(Base):
+    __tablename__ = "measurements"
+
+    id: Mapped[int] = mapped_column(BigInteger, primary_key=True, autoincrement=True)
+    subtask_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("recipe_subtasks.id"), nullable=False, index=True
+    )
+    version_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("recipe_versions.id"), nullable=False, index=True
+    )
+    measured_by: Mapped[int] = mapped_column(
+        Integer, ForeignKey("users.id"), nullable=False, index=True
+    )
+
+    # Measurement data
+    value: Mapped[float] = mapped_column(DECIMAL(12, 6), nullable=False)
+    pass_fail: Mapped[str] = mapped_column(
+        Enum("pass", "warning", "fail", name="pass_fail_enum"), nullable=False
+    )
+    deviation: Mapped[Optional[float]] = mapped_column(DECIMAL(12, 6), nullable=True)
+
+    # Traceability
+    lot_number: Mapped[Optional[str]] = mapped_column(String(100), nullable=True, index=True)
+    serial_number: Mapped[Optional[str]] = mapped_column(String(100), nullable=True, index=True)
+
+    # Input method
+    input_method: Mapped[str] = mapped_column(
+        Enum("usb_caliper", "manual", name="input_method_enum"),
+        nullable=False,
+        default="manual",
+    )
+
+    # Timestamp
+    measured_at: Mapped[datetime] = mapped_column(
+        DateTime, nullable=False, server_default=func.now(), index=True
+    )
+
+    # CSV sync flag
+    synced_to_csv: Mapped[bool] = mapped_column(Boolean, default=False)
+
+    def __repr__(self) -> str:
+        return f"<Measurement subtask={self.subtask_id} value={self.value} {self.pass_fail}>"