feat: add comprehensive tagging system with cooccurrence tracking

- Add tags, project_tags, and tag_cooccurrence tables with proper indexes
- Implement full CRUD API endpoints for tag management
- Add tag association endpoints for projects with automatic cooccurrence updates
- Include related tags and project filtering by tag functionality

migrations/20260106020134_add_tags_system.sql

@@ -0,0 +1,36 @@
+-- Tags table
+CREATE TABLE tags (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    slug TEXT NOT NULL UNIQUE,
+    name TEXT NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Indexes for tags
+CREATE INDEX idx_tags_slug ON tags(slug);
+
+-- Case-insensitive unique constraint on name
+CREATE UNIQUE INDEX idx_tags_name_lower ON tags(LOWER(name));
+
+-- Project-Tag junction table
+CREATE TABLE project_tags (
+    project_id UUID NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+    tag_id UUID NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+    PRIMARY KEY (project_id, tag_id)
+);
+
+-- Indexes for project_tags
+CREATE INDEX idx_project_tags_project_id ON project_tags(project_id);
+CREATE INDEX idx_project_tags_tag_id ON project_tags(tag_id);
+
+-- Tag cooccurrence matrix
+CREATE TABLE tag_cooccurrence (
+    tag_a UUID NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+    tag_b UUID NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+    count INTEGER NOT NULL DEFAULT 0,
+    PRIMARY KEY (tag_a, tag_b),
+    CHECK (tag_a < tag_b)
+);
+
+-- Index for reverse lookups
+CREATE INDEX idx_tag_cooccurrence_tag_b ON tag_cooccurrence(tag_b, tag_a);