BE-PR-003: Add Essential SQLAlchemy model

- New app/models/essential.py with Essential model and EssentialType enum
  (feature, improvement, refactor)
- Fields: id, essential_code (unique), proposal_id (FK to proposes),
  type, title, description, created_by_id (FK to users), created_at, updated_at
- Added essentials relationship to Proposal model (cascade delete-orphan)
- Added essentials table auto-migration in main.py _migrate_schema()
- Registered essential module import in startup()

app/models/proposal.py

@@ -1,5 +1,6 @@
 from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, Enum
 from sqlalchemy.ext.hybrid import hybrid_property
+from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
 from app.core.config import Base
 import enum
@@ -69,6 +70,14 @@ class Proposal(Base):
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), onupdate=func.now())
 
+    # ---- relationships -----------------------------------------------------
+    essentials = relationship(
+        "Essential",
+        foreign_keys="Essential.proposal_id",
+        cascade="all, delete-orphan",
+        lazy="select",
+    )
+
     # ---- convenience alias ------------------------------------------------
     @hybrid_property
     def proposal_code(self) -> str | None: