M6 1900: any-file attachments + audio memos (broaden beyond images)

A note can now carry any file, not just images — PDFs, documents, audio
memos, etc. "Dump anything" capture.

Backend:
- note_attachments.filename (migration 0019) records the original name for
  download + display.
- Upload drops the image-only mime gate: accepts any type, derives the
  storage extension from the filename, and enforces a DB-backed per-file
  cap — new setting max_attachment_mb (default 25, rule 25). App body
  ceiling raised 12→64 MB (also lifts the import-zip / sync-push limits);
  the per-file cap is the effective attachment limit.
- Serve sets Content-Disposition: images inline, everything else downloads
  with its original (header-sanitized) filename.
- Import (native + Keep Takeout) now brings in ANY attachment, not just
  images — completing the Keep audio-memo gap; preserves filename + sha256.
- Attachment metadata (delta feed + REST) carries filename.

Frontend:
- Editor renders attachments by kind: images inline (thumbnail), audio via
  an inline <audio> player, any other file as a download chip (paperclip +
  filename + size). File picker accepts any type; "Attach a file".
- Card previews the first image; non-image files show as compact chips.

Tests (DB-free): _safe_filename, _attachment_ext, _header_filename.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

alembic/versions/0019_attachment_filename.py

@@ -0,0 +1,25 @@
+"""note_attachments.filename (any-file attachments — M6 1900)
+
+Revision ID: 0019
+Revises: 0018
+Create Date: 2026-07-23
+
+Attachments broaden beyond images to any file; the original filename is recorded
+for the download name + display of non-image files. Nullable (old image rows have
+none — they render inline by mime anyway).
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0019"
+down_revision = "0018"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("note_attachments", sa.Column("filename", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("note_attachments", "filename")