feat(backend): migrations Postgres testées (users, devices, resources, operations, ocr_jobs) + migrate au boot

- golang-migrate embarqué via embed (db/migrations/*.sql), MigrateDatabase(url) au boot
- ids TEXT 32-hex avec CHECK ~ '^[0-9a-f]{32}$' ; operations idempotentes UNIQUE(device_id, operation_id)
- harness db/migrations_test.go (up → assertions schéma → down, skip si PG indisponible)
- DB dev fraîche vaultdrop_dev (la DB vaultdrop héberge un prototype V2 abandonné)

backend/db/migrations/000003_create_resources.up.sql

@@ -0,0 +1,21 @@
+CREATE TABLE resources (
+    resource_id TEXT PRIMARY KEY CHECK (resource_id ~ '^[0-9a-f]{32}$'),
+    type TEXT NOT NULL CHECK (type IN ('file', 'folder')),
+    name TEXT NOT NULL,
+    parent_id TEXT REFERENCES resources(resource_id) ON DELETE CASCADE,
+    owner_id TEXT NOT NULL REFERENCES devices(device_id) ON DELETE CASCADE,
+    content_hash VARCHAR(128),
+    size_bytes BIGINT DEFAULT 0,
+    mime_type VARCHAR(255),
+    extension VARCHAR(64),
+    category VARCHAR(50) CHECK (category IN ('photo', 'video', 'document', 'audio', 'other')),
+    taken_at TIMESTAMPTZ,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW(),
+    deleted_at TIMESTAMPTZ,
+    CONSTRAINT resources_unique_name_per_parent UNIQUE (parent_id, name)
+);
+
+CREATE INDEX idx_resources_owner ON resources(owner_id) WHERE deleted_at IS NULL;
+CREATE INDEX idx_resources_parent ON resources(parent_id);
+CREATE INDEX idx_resources_category ON resources(category) WHERE deleted_at IS NULL;