Remove proactive PyPI dependency caching feature

The background task queue for proactively caching package dependencies was
causing server instability and unnecessary growth. The PyPI proxy now only
caches packages on-demand when users request them.

Removed:
- PyPI cache worker (background task queue and worker pool)
- PyPICacheTask model and related database schema
- Cache management API endpoints (/pypi/cache/*)
- Background Jobs admin dashboard
- Dependency extraction and queueing logic

Kept:
- On-demand package caching (still works when users request packages)
- Async httpx for non-blocking downloads (prevents health check failures)
- URL-based cache lookups for deduplication

backend/app/models.py

@@ -803,70 +803,3 @@ class CachedUrl(Base):
         return hashlib.sha256(url.encode("utf-8")).hexdigest()
 
 
-class PyPICacheTask(Base):
-    """Task for caching a PyPI package and its dependencies.
-
-    Tracks the status of background caching operations with retry support.
-    Used by the PyPI proxy to ensure reliable dependency caching.
-    """
-
-    __tablename__ = "pypi_cache_tasks"
-
-    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
-
-    # What to cache
-    package_name = Column(String(255), nullable=False)
-    version_constraint = Column(String(255))
-
-    # Origin tracking
-    parent_task_id = Column(
-        UUID(as_uuid=True),
-        ForeignKey("pypi_cache_tasks.id", ondelete="SET NULL"),
-    )
-    depth = Column(Integer, nullable=False, default=0)
-    triggered_by_artifact = Column(
-        String(64),
-        ForeignKey("artifacts.id", ondelete="SET NULL"),
-    )
-
-    # Status
-    status = Column(String(20), nullable=False, default="pending")
-    attempts = Column(Integer, nullable=False, default=0)
-    max_attempts = Column(Integer, nullable=False, default=3)
-
-    # Results
-    cached_artifact_id = Column(
-        String(64),
-        ForeignKey("artifacts.id", ondelete="SET NULL"),
-    )
-    error_message = Column(Text)
-
-    # Timing
-    created_at = Column(DateTime(timezone=True), nullable=False, default=datetime.utcnow)
-    started_at = Column(DateTime(timezone=True))
-    completed_at = Column(DateTime(timezone=True))
-    next_retry_at = Column(DateTime(timezone=True))
-
-    # Relationships
-    parent_task = relationship(
-        "PyPICacheTask",
-        remote_side=[id],
-        backref="child_tasks",
-    )
-
-    __table_args__ = (
-        Index("idx_pypi_cache_tasks_status_retry", "status", "next_retry_at"),
-        Index("idx_pypi_cache_tasks_package_status", "package_name", "status"),
-        Index("idx_pypi_cache_tasks_parent", "parent_task_id"),
-        Index("idx_pypi_cache_tasks_triggered_by", "triggered_by_artifact"),
-        Index("idx_pypi_cache_tasks_cached_artifact", "cached_artifact_id"),
-        Index("idx_pypi_cache_tasks_depth_created", "depth", "created_at"),
-        CheckConstraint(
-            "status IN ('pending', 'in_progress', 'completed', 'failed')",
-            name="check_task_status",
-        ),
-        CheckConstraint("depth >= 0", name="check_depth_non_negative"),
-        CheckConstraint("attempts >= 0", name="check_attempts_non_negative"),
-    )
-
-