docs: update README with database schema and priority calculation strategy

.gitignore

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+/bin/
+/vendor/
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+*.test
+*.out
+/storage/*.db
+/storage/*.db-journal
+.env
+.idea/
+.vscode/
+*.swp
+*.swo
+.DS_Store
+

README.md

@@ -1 +1,9 @@
 # PieVR
+
+## Database Schema & Core Concepts
+- **Storage:** SQLite (`storage/` directory, ignored in `.gitignore`).
+- **Tables:**
+  - `questions`: stores core question data (`id`, `type`, `question_text`, `payload` as JSON, `active`, `last_asked_at`, `streak`, `times_asked`, `times_correct`, `created_at`).
+  - `tags`: tags for granular and cross-cutting topic selection (`id`, `name`).
+  - `question_tags`: many-to-many relationship between questions and tags.
+- **Priority & Spaced Repetition:** No hardcoded calendar intervals. Priorities and scheduling metrics are stored as raw counters (`streak`, `times_asked`, `last_asked_at`) in the database, while complex priority/decay formulas are calculated dynamically on the fly in Go.

internal/storage/schema.sql

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+-- PieVR Database Schema
+
+CREATE TABLE IF NOT EXISTS questions (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    type TEXT NOT NULL,
+    question_text TEXT NOT NULL,
+    payload TEXT NOT NULL,
+    active BOOLEAN DEFAULT 1,
+    last_asked_at DATETIME,
+    streak INTEGER DEFAULT 0,
+    times_asked INTEGER DEFAULT 0,
+    times_correct INTEGER DEFAULT 0,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS tags (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    name TEXT UNIQUE NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS question_tags (
+    question_id INTEGER REFERENCES questions(id) ON DELETE CASCADE,
+    tag_id INTEGER REFERENCES tags(id) ON DELETE CASCADE,
+    PRIMARY KEY (question_id, tag_id)
+);