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_member_mapping.py

@@ -0,0 +1,47 @@
+"""Maps ResolutionFlow users to CW members."""
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import String, DateTime, ForeignKey, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+
+from app.core.database import Base
+
+
+class PsaMemberMapping(Base):
+    __tablename__ = "psa_member_mappings"
+    __table_args__ = (
+        UniqueConstraint("psa_connection_id", "user_id", name="uq_psa_member_mapping_connection_user"),
+        UniqueConstraint("psa_connection_id", "external_member_id", name="uq_psa_member_mapping_connection_member"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    psa_connection_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("psa_connections.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    external_member_id: Mapped[str] = mapped_column(String(100), nullable=False)
+    external_member_name: Mapped[str] = mapped_column(String(200), nullable=False)
+    matched_by: Mapped[str] = mapped_column(String(50), nullable=False)  # 'auto_email', 'manual_admin', 'manual_self'
+    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
+    psa_connection = relationship("PsaConnection", foreign_keys=[psa_connection_id])
+    user = relationship("User", foreign_keys=[user_id])