feat(v5 slice 1): clamps schema + store helpers + snapshot

Migration 007 introduces the v5 routing primitive:
- clamps table (project-scoped, optional frame_id, excalidraw_id).
- cable_clamps join (cable_id, clamp_id, ord) with PK on (cable_id, ord)
  and UNIQUE (cable_id, clamp_id) to block a clamp visiting the same
  cable twice.

Store helpers in internal/db/clamps.go:
- CreateClamp / GetClamp / ListClamps / UpdateClamp / DeleteClamp —
  standard project-scoped CRUD. UpdateClamp uses FrameRef tri-state.
- AttachClampToCable — appends or inserts at a given ord. Mid-sequence
  inserts use a two-pass shift (bump by 10000, settle to ord+1) since
  SQLite UPDATE doesn't support ORDER BY and a single bulk +1 would
  collide with the UNIQUE (cable_id, ord) PK.
- DetachClampFromCable — removes the row then closes the gap.
- ReorderCableClamps — replaces the whole sequence in one tx.
- ListClampsForCable / ListCableClamps — read helpers.

Snapshot now carries clamps + cable_clamps arrays so the frontend can
hydrate everything in one call.

Tests cover create / update / cascade-delete / attach (append + insert
+ duplicate-rejected) / detach (gap closes) / reorder / snapshot.

internal/db/migrations/007_clamps.sql

@@ -0,0 +1,31 @@
+-- mCables v5 — cable routing via clamps. See docs/design.md §11.
+--
+-- A clamp is a physical anchor placed on the canvas. A cable's polyline
+-- runs from its `from` endpoint → its clamps in `ord` sequence → its
+-- `to` endpoint. Cables that share an ordered pair of consecutive
+-- clamps are visibly bundled along that segment (computed live by the
+-- frontend; no detection pass).
+
+CREATE TABLE clamps (
+    id              INTEGER PRIMARY KEY,
+    project_id      INTEGER NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+    x               REAL NOT NULL,
+    y               REAL NOT NULL,
+    label           TEXT NOT NULL DEFAULT '',
+    frame_id        INTEGER REFERENCES frames(id) ON DELETE SET NULL,
+    excalidraw_id   TEXT,
+    created_at      TEXT NOT NULL DEFAULT (datetime('now')),
+    updated_at      TEXT NOT NULL DEFAULT (datetime('now')),
+    UNIQUE (project_id, excalidraw_id)
+);
+CREATE INDEX clamps_project_idx ON clamps(project_id);
+CREATE INDEX clamps_frame_idx   ON clamps(frame_id);
+
+CREATE TABLE cable_clamps (
+    cable_id        INTEGER NOT NULL REFERENCES cables(id) ON DELETE CASCADE,
+    clamp_id        INTEGER NOT NULL REFERENCES clamps(id) ON DELETE CASCADE,
+    ord             INTEGER NOT NULL,            -- 1-based along from→to
+    PRIMARY KEY (cable_id, ord),
+    UNIQUE (cable_id, clamp_id)
+);
+CREATE INDEX cable_clamps_clamp_idx ON cable_clamps(clamp_id);