feat(PROJ-9): implement labels backend - DB schema, labelstore, API handlers, Xapian integration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/storage/migrations/012_labels.sql

@@ -0,0 +1,32 @@
+-- PROJ-9: Labels, email-label assignments, and auto-label rules.
+-- This file is documentation; the actual schema is applied by labelstore.initSchema().
+
+CREATE TABLE IF NOT EXISTS labels (
+    id         BIGSERIAL PRIMARY KEY,
+    name       VARCHAR(100) NOT NULL,
+    color      VARCHAR(7) NOT NULL DEFAULT '#6366f1',
+    owner_id   BIGINT REFERENCES users(id) ON DELETE CASCADE,
+    tenant_id  BIGINT REFERENCES tenants(id) ON DELETE CASCADE,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    UNIQUE(name, owner_id, tenant_id)
+);
+
+CREATE TABLE IF NOT EXISTS email_labels (
+    email_id    VARCHAR(64) NOT NULL,
+    label_id    BIGINT NOT NULL REFERENCES labels(id) ON DELETE CASCADE,
+    assigned_at TIMESTAMPTZ DEFAULT NOW(),
+    assigned_by VARCHAR(20) NOT NULL DEFAULT 'user',
+    PRIMARY KEY (email_id, label_id)
+);
+
+CREATE TABLE IF NOT EXISTS label_rules (
+    id               BIGSERIAL PRIMARY KEY,
+    condition_field  VARCHAR(30) NOT NULL,
+    condition_value  VARCHAR(255) NOT NULL,
+    label_id         BIGINT NOT NULL REFERENCES labels(id) ON DELETE CASCADE,
+    tenant_id        BIGINT REFERENCES tenants(id) ON DELETE CASCADE,
+    created_at       TIMESTAMPTZ DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_email_labels_label_id ON email_labels(label_id);
+CREATE INDEX IF NOT EXISTS idx_label_rules_tenant_id ON label_rules(tenant_id);