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/essential.py

@@ -0,0 +1,59 @@
+"""Essential model — actionable items under a Proposal.
+
+Each Essential represents one deliverable scope item (feature, improvement,
+or refactor).  When a Proposal is accepted, every Essential is converted into
+a corresponding ``story/*`` task under the chosen Milestone.
+
+See: NEXT_WAVE_DEV_DIRECTION.md §8.5
+"""
+
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, Enum
+from sqlalchemy.sql import func
+from app.core.config import Base
+import enum
+
+
+class EssentialType(str, enum.Enum):
+    FEATURE = "feature"
+    IMPROVEMENT = "improvement"
+    REFACTOR = "refactor"
+
+
+class Essential(Base):
+    __tablename__ = "essentials"
+
+    id = Column(Integer, primary_key=True, index=True)
+
+    essential_code = Column(
+        String(64),
+        nullable=False,
+        unique=True,
+        index=True,
+        comment="Unique human-readable code, e.g. PROJ:E00001",
+    )
+
+    proposal_id = Column(
+        Integer,
+        ForeignKey("proposes.id"),  # FK targets the actual DB table name
+        nullable=False,
+        comment="Owning Proposal",
+    )
+
+    type = Column(
+        Enum(EssentialType, values_callable=lambda x: [e.value for e in x]),
+        nullable=False,
+        comment="Essential type: feature | improvement | refactor",
+    )
+
+    title = Column(String(255), nullable=False, comment="Short title")
+    description = Column(Text, nullable=True, comment="Detailed description")
+
+    created_by_id = Column(
+        Integer,
+        ForeignKey("users.id"),
+        nullable=True,
+        comment="Author of the essential",
+    )
+
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())