feat(classify): add rule-based document classification engine, task, and API

Implements the classify pipeline step with:
- Classification rules engine (app/utils/classification_rules.py) with
  pre-built categories (invoice, contract, receipt, letter, report,
  bank_statement, tax_document, insurance, payslip) and support for
  filename patterns, content keywords, and metadata matching rules
- Celery task (app/tasks/classify_document.py) that runs as a pipeline step
- CRUD API (app/api/classification_rules.py) for managing custom rules
- ClassificationRuleModel in app/models.py with migration 027
- Updated pipeline step config_schema and stage mapping
- Comprehensive tests for engine, API, and task

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/models.py

@@ -833,3 +833,48 @@ class ScheduledJob(Base):
 
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+
+class ClassificationRuleModel(Base):
+    """Custom document classification rule.
+
+    Rules are evaluated during the ``classify`` pipeline step to assign a
+    category to a document.  System-wide rules have ``owner_id IS NULL``;
+    user-specific rules belong to a single owner.
+    """
+
+    __tablename__ = "classification_rules"
+
+    id = Column(Integer, primary_key=True, index=True)
+
+    # NULL = system-wide rule visible to all users.
+    owner_id = Column(String, nullable=True, index=True)
+
+    # Human-readable rule name (unique per owner).
+    name = Column(String(255), nullable=False)
+
+    # Target category (e.g. "invoice", "contract", "receipt").
+    category = Column(String(100), nullable=False, index=True)
+
+    # Rule type: "filename_pattern", "content_keyword", or "metadata_match".
+    rule_type = Column(String(50), nullable=False)
+
+    # The matching pattern:
+    #   - filename_pattern: a regex
+    #   - content_keyword: pipe-separated keywords
+    #   - metadata_match: "field=value"
+    pattern = Column(String(1000), nullable=False)
+
+    # Higher priority rules are evaluated first (default 0).
+    priority = Column(Integer, nullable=False, default=0)
+
+    # Whether pattern matching is case-sensitive.
+    case_sensitive = Column(Boolean, nullable=False, default=False)
+
+    # Disabled rules are skipped during classification.
+    enabled = Column(Boolean, nullable=False, default=True)
+
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    __table_args__ = (UniqueConstraint("owner_id", "name", name="uq_classification_rules_owner_name"),)