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()