feat(llm): RAG knowledge base over clinical guidelines with cited extractive answers

- docs/source/rag: PDF sources (KR PTSD 2023, KR depression 2024, meds module spec),
  pdf->txt converter (tables as markdown, page markers, hand-transcribed figures),
  word-level check against pdftotext; fixes HAM-D/MADRS fake headers and split rows
- config/rag: corpus txt + sources.yaml (documents, audiences, retrieval params, synonyms)
- app/llm/rag: Snowball stemmer, section-aware chunker with page citations,
  BM25 + synonym expansion + densest-passage reranking, extractive answer()
  with coverage gate, patient dose/prescribing filter, optional LLM rephrasing
  rejected when it adds content; CLI python -m app.llm.rag
- tests: gold retrieval set (hit@3 >= 15/18), off-topic, safety, faithfulness
- ADR-011

Refs #60

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/llm/rag/stemmer.py

@@ -0,0 +1,112 @@
+"""Russian Snowball stemmer (https://snowballstem.org/algorithms/russian/stemmer.html).
+
+Self-contained because `pyproject.toml` is a frozen contract (#34): no new dependencies for the RAG.
+Latin words and digits pass through lowercased; "ё" is folded to "е".
+"""
+
+import re
+from functools import lru_cache
+
+_VOWELS = set("аеиоуыэюя")
+
+# (suffixes, needs_a_ya): group-1 suffixes count only when preceded by "а"/"я" (which stays in the stem).
+_PERFECTIVE_GERUND = ((("вшись", "вши", "в"), True), (("ившись", "ывшись", "ивши", "ывши", "ив", "ыв"), False))
+_REFLEXIVE = ((("ся", "сь"), False),)
+_ADJECTIVE = (
+    (
+        (
+            "ими", "ыми", "его", "ого", "ему", "ому", "ее", "ие", "ые", "ое", "ей", "ий", "ый", "ой", "ем",
+            "им", "ым", "ом", "их", "ых", "ую", "юю", "ая", "яя", "ою", "ею",
+        ),
+        False,
+    ),
+)  # fmt: skip
+_PARTICIPLE = ((("ем", "нн", "вш", "ющ", "щ"), True), (("ивш", "ывш", "ующ"), False))
+_VERB = (
+    (("ла", "на", "ете", "йте", "ли", "й", "л", "ем", "н", "ло", "но", "ет", "ют", "ны", "ть", "ешь", "нно"), True),
+    (
+        (
+            "ила", "ыла", "ена", "ейте", "уйте", "ите", "или", "ыли", "ей", "уй", "ил", "ыл", "им", "ым", "ен",
+            "ило", "ыло", "ено", "ят", "ует", "уют", "ит", "ыт", "ены", "ить", "ыть", "ишь", "ую", "ю",
+        ),
+        False,
+    ),
+)  # fmt: skip
+_NOUN = (
+    (
+        (
+            "иями", "ями", "ами", "ией", "иям", "ием", "иях", "ев", "ов", "ие", "ье", "еи", "ии", "ей", "ой",
+            "ий", "ям", "ем", "ам", "ом", "ах", "ях", "ию", "ью", "ия", "ья", "а", "е", "и", "й", "о", "у",
+            "ы", "ь", "ю", "я",
+        ),
+        False,
+    ),
+)  # fmt: skip
+_DERIVATIONAL = ((("ость", "ост"), False),)
+_SUPERLATIVE = ((("ейше", "ейш"), False),)
+
+_CYRILLIC_WORD = re.compile(r"^[а-я]+$")
+
+
+def _strip(word: str, start: int, groups: tuple[tuple[tuple[str, ...], bool], ...]) -> str | None:
+    """Remove the longest matching suffix lying in word[start:], or `None` if none matches."""
+    best: tuple[int, str] | None = None
+    for suffixes, needs_a_ya in groups:
+        for suffix in suffixes:
+            if not word.endswith(suffix) or len(word) - len(suffix) < start:
+                continue
+            if needs_a_ya and (len(word) - len(suffix) - 1 < start or word[-len(suffix) - 1] not in "ая"):
+                continue
+            if best is None or len(suffix) > best[0]:
+                best = (len(suffix), word[: -len(suffix)])
+    return None if best is None else best[1]
+
+
+def _regions(word: str) -> tuple[int, int]:
+    """RV (after the first vowel) and R2 start indices."""
+    rv = next((i + 1 for i, ch in enumerate(word) if ch in _VOWELS), len(word))
+
+    def after_vc(start: int) -> int:
+        for i in range(start + 1, len(word)):
+            if word[i] not in _VOWELS and word[i - 1] in _VOWELS:
+                return i + 1
+        return len(word)
+
+    return rv, after_vc(after_vc(0))
+
+
+@lru_cache(maxsize=65536)
+def stem(word: str) -> str:
+    word = word.lower().replace("ё", "е")
+    if not _CYRILLIC_WORD.match(word):
+        return word
+    rv, r2 = _regions(word)
+
+    # Step 1
+    out = _strip(word, rv, _PERFECTIVE_GERUND)
+    if out is None:
+        word = _strip(word, rv, _REFLEXIVE) or word
+        out = _strip(word, rv, _ADJECTIVE)
+        if out is not None:
+            out = _strip(out, rv, _PARTICIPLE) or out
+        else:
+            out = _strip(word, rv, _VERB)
+            if out is None:
+                out = _strip(word, rv, _NOUN)
+    word = out if out is not None else word
+
+    # Step 2
+    if word.endswith("и") and len(word) - 1 >= rv:
+        word = word[:-1]
+    # Step 3
+    word = _strip(word, r2, _DERIVATIONAL) or word
+    # Step 4
+    if word.endswith("нн") and len(word) - 1 >= rv:
+        return word[:-1]
+    sup = _strip(word, rv, _SUPERLATIVE)
+    if sup is not None:
+        word = sup
+        return word[:-1] if word.endswith("нн") else word
+    if word.endswith("ь") and len(word) - 1 >= rv:
+        word = word[:-1]
+    return word