feat: ConnectWise PSA integration (#106)

PSA abstraction layer with provider pattern, ConnectWise integration (connection management, ticket linking, note posting, status updates, member mapping), Integrations page UI, Fernet credential encryption, in-memory TTL cache, 6 DB migrations, ConnectWise API reference docs.

backend/app/models/psa_connection.py

@@ -0,0 +1,48 @@
+"""PSA connection model — one per account."""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional
+
+from sqlalchemy import String, DateTime, Boolean, Text, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+
+from app.core.database import Base
+
+
+class PsaConnection(Base):
+    __tablename__ = "psa_connections"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        unique=True,
+        index=True,
+    )
+    provider: Mapped[str] = mapped_column(String(50), nullable=False)
+    display_name: Mapped[str] = mapped_column(String(100), nullable=False)
+    site_url: Mapped[str] = mapped_column(String(255), nullable=False)
+    company_id: Mapped[str] = mapped_column(String(100), nullable=False)
+    credentials_encrypted: Mapped[str] = mapped_column(Text, nullable=False)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    last_validated_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        default=lambda: datetime.now(timezone.utc),
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        default=lambda: datetime.now(timezone.utc),
+        onupdate=lambda: datetime.now(timezone.utc),
+    )
+
+    # Relationships
+    account = relationship("Account", back_populates="psa_connection")