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/main.py

@@ -259,6 +259,27 @@ def _migrate_schema():
         if _has_table(db, "server_states") and not _has_column(db, "server_states", "nginx_sites_json"):
             db.execute(text("ALTER TABLE server_states ADD COLUMN nginx_sites_json TEXT NULL"))
 
+        # --- essentials table (BE-PR-003) ---
+        if not _has_table(db, "essentials"):
+            db.execute(text("""
+                CREATE TABLE essentials (
+                    id INTEGER NOT NULL AUTO_INCREMENT,
+                    essential_code VARCHAR(64) NOT NULL,
+                    proposal_id INTEGER NOT NULL,
+                    type ENUM('feature','improvement','refactor') NOT NULL,
+                    title VARCHAR(255) NOT NULL,
+                    description TEXT NULL,
+                    created_by_id INTEGER NULL,
+                    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+                    updated_at DATETIME NULL ON UPDATE CURRENT_TIMESTAMP,
+                    PRIMARY KEY (id),
+                    UNIQUE INDEX idx_essentials_code (essential_code),
+                    INDEX idx_essentials_proposal_id (proposal_id),
+                    CONSTRAINT fk_essentials_proposal_id FOREIGN KEY (proposal_id) REFERENCES proposes(id),
+                    CONSTRAINT fk_essentials_created_by_id FOREIGN KEY (created_by_id) REFERENCES users(id)
+                ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
+            """))
+
         db.commit()
     except Exception as e:
         db.rollback()
@@ -293,7 +314,7 @@ def _sync_default_user_roles(db):
 @app.on_event("startup")
 def startup():
     from app.core.config import Base, engine, SessionLocal
-    from app.models import models, webhook, apikey, activity, milestone, notification, worklog, monitor, role_permission, task, support, meeting, proposal, propose
+    from app.models import models, webhook, apikey, activity, milestone, notification, worklog, monitor, role_permission, task, support, meeting, proposal, propose, essential
     Base.metadata.create_all(bind=engine)
     _migrate_schema()
 

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())

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: