feat(uploads): direct-to-B2 presigned uploads with content-length-range policy

Replaces the multipart-via-API path for image uploads with a three-step
direct-to-storage flow:

  1. Client POSTs /api/uploads/presign with content_length + content_type;
     server validates size (10 MB cap), mime allow-list per category, rate
     limit (50/hour/user via Redis sliding window), and concurrent unclaimed
     cap (10 in-flight per user). On success it persists a pending_uploads
     row, signs an S3 POST policy with content-length-range bound to the
     claimed length ±256 bytes, and returns the URL+fields.
  2. Client POSTs the bytes directly to B2 using the signed policy. B2
     enforces size, content-type, and key match before accepting.
  3. Client passes upload_ids[] to /api/task-completions/ or /api/documents/.
     Service HEADs each B2 object, verifies size matches expected_bytes
     within slack, marks pending_uploads claimed_at, and creates the
     associated TaskCompletionImage / DocumentImage rows.

Bytes never traverse our API server. The 1 MB Echo BodyLimit middleware
that was rejecting all task-completion image uploads becomes irrelevant
for this path. Existing multipart endpoints stay functional alongside,
soak-testing the new path before legacy removal.

Cleanup:
  - cmd/worker registers a new hourly cron (TypeUploadCleanup, "30 * * * *")
    that reaps pending_uploads where claimed_at IS NULL AND expires_at < NOW().
    Reaps both the B2 object and the row.
  - B2 bucket lifecycle rule on `uploads/` prefix (7 days hide → 1 day delete)
    documented in deploy-k3s/manifests/b2-lifecycle.md as a backstop.

Schema:
  - migrations/000002_pending_uploads.sql adds the table + partial index for
    cleanup + nullable pending_upload_id FKs on task_taskcompletionimage and
    task_documentimage.

Policy (single tier, no free/pro split):
  - 10 MB cap per upload
  - 50 presigns/hour/user
  - 10 concurrent unclaimed uploads/user
  - allow-list: jpeg/png/heic/heif/webp for image categories;
    + pdf for document_file

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/dto/responses/upload.go

@@ -0,0 +1,28 @@
+package responses
+
+// PresignUploadResponse is what /api/uploads/presign returns to the client.
+//
+// The client uses URL + Fields to build a multipart/form-data POST directly
+// to S3-compatible storage (B2). Once the upload completes, the client calls
+// the relevant entity-creation endpoint (POST /api/task-completions/, POST
+// /api/documents/) with `upload_ids: [Id]` to claim and attach the object.
+type PresignUploadResponse struct {
+	// ID is the pending_uploads.id the client passes back via upload_ids[].
+	ID uint `json:"id"`
+
+	// URL is the storage endpoint to POST to (no query string).
+	URL string `json:"upload_url"`
+
+	// Fields are the form fields (policy, signature, key, etc.) that must be
+	// submitted with the multipart form. The file part must be named "file"
+	// and come last per S3 POST policy rules.
+	Fields map[string]string `json:"fields"`
+
+	// Key is the object key chosen by the server. Echoed for client logging
+	// and debugging; the canonical reference is via ID.
+	Key string `json:"key"`
+
+	// ExpiresAt is when the signed URL stops working. Clients should retry
+	// with a fresh presign rather than relying on long-lived URLs.
+	ExpiresAt string `json:"expires_at"`
+}